diff --git a/dom/filesystem/GetDirectoryListingTask.cpp b/dom/filesystem/GetDirectoryListingTask.cpp
index 1d4f77b3b8..8fdfb4a5bb 100644
--- a/dom/filesystem/GetDirectoryListingTask.cpp
+++ b/dom/filesystem/GetDirectoryListingTask.cpp
@@ -340,9 +340,14 @@ GetDirectoryListingTaskParent::IOWork()
     nsCOMPtr<nsIFile> currFile = do_QueryInterface(supp);
     MOZ_ASSERT(currFile);
 
-    bool isSpecial, isFile;
-    if (NS_WARN_IF(NS_FAILED(currFile->IsSpecial(&isSpecial))) ||
-        isSpecial) {
+    bool isLink, isSpecial, isFile;
+    if (NS_WARN_IF(NS_FAILED(currFile->IsSymlink(&isLink)) ||
+                   NS_FAILED(currFile->IsSpecial(&isSpecial))) ||
+        // Although we allow explicit individual selection of symlinks via the
+        // file picker, we do not process symlinks in directory traversal.  Our
+        // specific policy decision is documented at
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1813299#c20
+        isLink || isSpecial) {
       continue;
     }
     if (NS_WARN_IF(NS_FAILED(currFile->IsFile(&isFile)) ||
diff --git a/dom/filesystem/GetFilesHelper.cpp b/dom/filesystem/GetFilesHelper.cpp
index 563ef60b4a..fb3cf7a24d 100644
--- a/dom/filesystem/GetFilesHelper.cpp
+++ b/dom/filesystem/GetFilesHelper.cpp
@@ -333,13 +333,8 @@ GetFilesHelperBase::ExploreDirectory(const nsAString& aDOMPath, nsIFile* aFile)
     return NS_OK;
   }
 
-  nsresult rv = AddExploredDirectory(aFile);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
-
   nsCOMPtr<nsISimpleEnumerator> entries;
-  rv = aFile->GetDirectoryEntries(getter_AddRefs(entries));
+  nsresult rv = aFile->GetDirectoryEntries(getter_AddRefs(entries));
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return rv;
   }
@@ -361,7 +356,12 @@ GetFilesHelperBase::ExploreDirectory(const nsAString& aDOMPath, nsIFile* aFile)
     bool isLink, isSpecial, isFile, isDir;
     if (NS_WARN_IF(NS_FAILED(currFile->IsSymlink(&isLink)) ||
                    NS_FAILED(currFile->IsSpecial(&isSpecial))) ||
-        isSpecial) {
+        isSpecial ||
+        // Although we allow explicit individual selection of symlinks via the
+        // file picker, we do not process symlinks in directory traversal.  Our
+        // specific policy decision is documented at
+        // https://bugzilla.mozilla.org/show_bug.cgi?id=1813299#c20
+        isLink) {
       continue;
     }
 
@@ -371,11 +371,6 @@ GetFilesHelperBase::ExploreDirectory(const nsAString& aDOMPath, nsIFile* aFile)
       continue;
     }
 
-    // We don't want to explore loops of links.
-    if (isDir && isLink && !ShouldFollowSymLink(currFile)) {
-      continue;
-    }
-
     // The new domPath
     nsAutoString domPath;
     domPath.Assign(aDOMPath);
@@ -415,69 +410,6 @@ GetFilesHelperBase::ExploreDirectory(const nsAString& aDOMPath, nsIFile* aFile)
   return NS_OK;
 }
 
-nsresult
-GetFilesHelperBase::AddExploredDirectory(nsIFile* aDir)
-{
-  nsresult rv;
-
-#ifdef DEBUG
-  bool isDir;
-  rv = aDir->IsDirectory(&isDir);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
-
-  MOZ_ASSERT(isDir, "Why are we here?");
-#endif
-
-  bool isLink;
-  rv = aDir->IsSymlink(&isLink);
-  if (NS_WARN_IF(NS_FAILED(rv))) {
-    return rv;
-  }
-
-  nsAutoCString path;
-
-  if (!isLink) {
-    nsAutoString path16;
-    rv = aDir->GetPath(path16);
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return rv;
-    }
-
-    path = NS_ConvertUTF16toUTF8(path16);
-  } else {
-    rv = aDir->GetNativeTarget(path);
-    if (NS_WARN_IF(NS_FAILED(rv))) {
-      return rv;
-    }
-  }
-
-  mExploredDirectories.PutEntry(path);
-  return NS_OK;
-}
-
-bool
-GetFilesHelperBase::ShouldFollowSymLink(nsIFile* aDir)
-{
-#ifdef DEBUG
-  bool isLink, isDir;
-  if (NS_WARN_IF(NS_FAILED(aDir->IsSymlink(&isLink)) ||
-                 NS_FAILED(aDir->IsDirectory(&isDir)))) {
-    return false;
-  }
-
-  MOZ_ASSERT(isLink && isDir, "Why are we here?");
-#endif
-
-  nsAutoCString targetPath;
-  if (NS_WARN_IF(NS_FAILED(aDir->GetNativeTarget(targetPath)))) {
-    return false;
-  }
-
-  return !mExploredDirectories.Contains(targetPath);
-}
-
 void
 GetFilesHelper::ResolveOrRejectPromise(Promise* aPromise)
 {
diff --git a/dom/filesystem/GetFilesHelper.h b/dom/filesystem/GetFilesHelper.h
index 4afd41d7e0..b6c58ef396 100644
--- a/dom/filesystem/GetFilesHelper.h
+++ b/dom/filesystem/GetFilesHelper.h
@@ -9,10 +9,8 @@
 #include "mozilla/Mutex.h"
 #include "mozilla/RefPtr.h"
 #include "mozilla/dom/File.h"
-#include "nsClassHashtable.h"
 #include "nsCycleCollectionTraversalCallback.h"
 #include "nsTArray.h"
-#include "nsTHashtable.h"
 #include "nsThreadUtils.h"
 
 class nsIGlobalObject;
@@ -57,17 +55,10 @@ protected:
   nsresult
   ExploreDirectory(const nsAString& aDOMPath, nsIFile* aFile);
 
-  nsresult
-  AddExploredDirectory(nsIFile* aDirectory);
-
-  bool
-  ShouldFollowSymLink(nsIFile* aDirectory);
-
   bool mRecursiveFlag;
 
   // We populate this array in the I/O thread with the BlobImpl.
   FallibleTArray<RefPtr<BlobImpl>> mTargetBlobImplArray;
-  nsTHashtable<nsCStringHashKey> mExploredDirectories;
 };
 
 // Retrieving the list of files can be very time/IO consuming. We use this
diff --git a/dom/filesystem/tests/filesystem_commons.js b/dom/filesystem/tests/filesystem_commons.js
index 4f7234121e..c8bd9ac9fc 100644
--- a/dom/filesystem/tests/filesystem_commons.js
+++ b/dom/filesystem/tests/filesystem_commons.js
@@ -38,6 +38,10 @@ function test_getFilesAndDirectories(aDirectory, aRecursive, aNext) {
           if (data[i] instanceof File) {
             is(data[i].webkitRelativePath, createRelativePath(dir, data[i]), "File.webkitRelativePath should be called: parentdir.path + '/' + file.name: " + data[i].webkitRelativePath);
           }
+          ok(
+            !data[i].webkitRelativePath.endsWith("symlink.txt"),
+            "We should never see a path ending with symlink.txt, our symlink sentinel."
+          );
         }
       }
     );
diff --git a/dom/filesystem/tests/script_fileList.js b/dom/filesystem/tests/script_fileList.js
index 89fd04cabe..dedc61a448 100644
--- a/dom/filesystem/tests/script_fileList.js
+++ b/dom/filesystem/tests/script_fileList.js
@@ -36,6 +36,15 @@ function createTreeFile(depth, parent) {
   if (depth == 0) {
     nextFile.append('file.txt');
     nextFile.create(Components.interfaces.nsIFile.NORMAL_FILE_TYPE, 0o600);
+
+#ifdef XP_UNIX
+    // It's not possible to create symlinks on windows by default or on our
+    // Android platforms, so we can't create the symlink file there.  Our
+    // callers that care are aware of this.
+    var linkFile = parent.clone();
+    linkFile.append("symlink.txt");
+    createSymLink(nextFile.path, linkFile.path);
+#endif
   } else {
     nextFile.append('subdir' + depth);
     nextFile.createUnique(Components.interfaces.nsIFile.DIRECTORY_TYPE, 0o700);
@@ -84,6 +93,15 @@ function createTestFile() {
   file2.append('bar.txt');
   file2.create(Components.interfaces.nsIFile.NORMAL_FILE_TYPE, 0o600);
 
+#ifdef XP_UNIX
+  // It's not possible to create symlinks on windows by default or on our
+  // Android platforms, so we can't create the symlink file there.  Our
+  // callers that care are aware of this.
+  var linkFile = dir.clone();
+  linkFile.append("symlink.txt");
+  createSymLink(file1.path, linkFile.path);
+#endif 
+
   return tmpFile;
 }
 
@@ -127,3 +145,14 @@ addMessageListener("file.open", function (e) {
     file: File.createFromNsIFile(testFile)
   });
 });
+
+addMessageListener("symlink.open", function (e) {
+  let testDir = createTestFile();
+  let testFile = testDir.clone();
+  testFile.append("subdir");
+  testFile.append("symlink.txt");
+
+  File.createFromNsIFile(testFile).then(function (file) {
+    sendAsyncMessage("symlink.opened", { dir: testDir.path, file });
+  });
+});
diff --git a/dom/filesystem/tests/test_webkitdirectory.html b/dom/filesystem/tests/test_webkitdirectory.html
index 591619e45b..46c728718f 100644
--- a/dom/filesystem/tests/test_webkitdirectory.html
+++ b/dom/filesystem/tests/test_webkitdirectory.html
@@ -9,11 +9,26 @@
 <body>
 <input id="inputFileWebkitDirectory" type="file" webkitdirectory></input>
 <input id="inputFileWebkitDirectoryAndDirectory" type="file" webkitdirectory allowdirs></input>
+<input id="inputFileWebkitFile" type="file"></input>
 <input id="inputFileDirectory" type="file" allowdirs></input>
 <input id="inputFileDirectoryChange" type="file" webkitdirectory></input>
 
 <script type="application/javascript;version=1.7">
 
+// Populate the given input type=file `aInputFile`'s `files` attribute by:
+// - loading `script_fileList.js` in the parent process
+// - telling it to generate the "test" template directory pattern which will
+//   create "foo.txt", "subdir/bar.txt", and if symlinks are available on the
+//   platform, "symlink.txt" which will be a symlink to "foo.txt".  (Note that
+//   we explicitly expect the symlink to be filtered out if generated, and
+//   during the enhancement of the test we verified the file was created on
+//   linux by running the test before fixing the GetFilesHelper logic to filter
+//   the symlink out and verifying the subsequent `test_fileList` check failed.)
+// - Triggering the mock file picker with the base directory of the "test"
+//   template directory.
+//
+// It's expected that `test_fileList` will be used after this step completes in
+// order to validate the results.
 function populateInputFile(aInputFile) {
   var url = SimpleTest.getTestFileURL("script_fileList.js");
   var script = SpecialPowers.loadChromeScript(url);
@@ -52,6 +67,8 @@ function checkFile(file, fileList, dirName) {
   ok(false, "File not found.");
 }
 
+// Validate the contents of the given input type=file `aInputFile`'s' `files`
+// property against the expected list of files `aWhat`.
 function test_fileList(aInputFile, aWhat) {
   var input = document.getElementById(aInputFile);
   var fileList = input.files;
@@ -70,6 +87,65 @@ function test_fileList(aInputFile, aWhat) {
   next();
 }
 
+// Verify that we can explicitly select a symlink and it will not be filtered
+// out.  This is really a verification that GetFileHelper's file-handling logic
+// https://searchfox.org/mozilla-central/rev/065102493dfc49234120c37fc6a334a5b1d86d9e/dom/filesystem/GetFilesHelper.cpp#81-86
+// does not proactively take an action to filter out a selected symlink.
+//
+// This is a glass box test that is not entirely realistic for our actual system
+// file pickers but does reflect what will happen in the drag-and-drop case
+// for `HTMLInputElement::MozSetDndFilesAndDirectories` and this helps ensure
+// that future implementation changes will behave as expected.  Specifically,
+// the presence of webkitdirectory will result in the file picker using
+// `modeGetFolder` which will only allow selection of a directory and forbid
+// file selection.
+//
+// This test explicitly does not validate HTMLInputElement's non-webkitdirectory
+// file selection mechanism because it does not involve GetFileHelper.
+async function test_individualSymlink(aInputFile) {
+  const input = document.getElementById(aInputFile);
+
+  // -- Create the symlink and get a `File` instance pointing at it.
+  const url = SimpleTest.getTestFileURL("script_fileList.js");
+  const script = SpecialPowers.loadChromeScript(url);
+
+  let opened = new Promise(resolve => script.addMessageListener("symlink.opened", resolve));
+  script.sendAsyncMessage("symlink.open", {});
+  let { dir, file: symlinkFile } = await opened;
+  info(`symlink.open provided dir: ${dir}`)
+
+  // -- Have the picker pick it
+  var MockFilePicker = SpecialPowers.MockFilePicker;
+  MockFilePicker.init(window, "A Mock File Picker", SpecialPowers.Ci.nsIFilePicker.modeOpen);
+
+  MockFilePicker.displayDirectory = dir;
+  let pickerShown = new Promise(resolve => {
+    MockFilePicker.showCallback = function() {
+      // This is where we are diverging from a realistic scenario in order to get
+      // the expected coverage.
+      MockFilePicker.setFiles([symlinkFile]);
+      resolve();
+    }
+  });
+  MockFilePicker.returnValue = MockFilePicker.returnOK;
+
+  let changeEvent = waitForEvent(input, "change");
+
+  input.click();
+
+  await pickerShown;
+  await changeEvent;
+
+  MockFilePicker.cleanup();
+  script.destroy();
+
+  // -- Verify that we see the symlink.
+  let fileList = input.files;
+  is(fileList.length, 1, "There should be 1 file.");
+  is(fileList[0].name, "symlink.txt", "The file should be the symlink.");
+  next();
+}
+
 function test_webkitdirectory_attribute() {
   var a = document.createElement("input");
   a.setAttribute("type", "file");
@@ -169,6 +245,16 @@ var tests = [
   function() { test_fileList('inputFileWebkitDirectory', testDirData) },
   function() { test_fileList('inputFileWebkitDirectoryAndDirectory', testDirData) },
   function() { test_fileList('inputFileDirectory', null); },
+  
+  function() {
+#ifdef XP_WIN    // Symlinks are not available on Windows and so will not be created.
+    info("Skipping individual symlink check on Windows and Android.");
+    next();
+    return;
+#endif
+    test_individualSymlink("inputFileWebkitFile").catch(err => ok(false, `Problem in symlink case: ${err}`));
+  },
+
 
   test_webkitdirectory_attribute,
 
diff --git a/js/public/Proxy.h b/js/public/Proxy.h
index e493f522c6..981adc853c 100644
--- a/js/public/Proxy.h
+++ b/js/public/Proxy.h
@@ -588,6 +588,15 @@ class JS_FRIEND_API(AutoEnterPolicy)
     inline void recordLeave() {}
 #endif
 
+  private:
+    // This operator needs to be deleted explicitly, otherwise Visual C++ will
+    // create it automatically when it is part of the export JS API. In that
+    // case, compile would fail because HandleId is not allowed to be assigned
+    // and consequently instantiation of assign operator of mozilla::Maybe
+    // would fail. See bug 1325351 comment 16. Copy constructor is removed at
+    // the same time for consistency.
+    AutoEnterPolicy(const AutoEnterPolicy&) = delete;
+    AutoEnterPolicy& operator=(const AutoEnterPolicy&) = delete;
 };
 
 #ifdef JS_DEBUG
diff --git a/mfbt/Maybe.h b/mfbt/Maybe.h
index 79df8d2518..bc123b047f 100644
--- a/mfbt/Maybe.h
+++ b/mfbt/Maybe.h
@@ -102,16 +102,11 @@ public:
   }
 
   /**
-   * Maybe<T*> can be copy-constructed from a Maybe<U*> if U* and T* are
-   * compatible, or from Maybe<decltype(nullptr)>.
+   * Maybe<T> can be copy-constructed from a Maybe<U> if U is convertible to T.
    */
   template<typename U,
            typename =
-             typename std::enable_if<std::is_pointer<T>::value &&
-                                     (std::is_same<U, decltype(nullptr)>::value ||
-                                      (std::is_pointer<U>::value &&
-                                       std::is_base_of<typename std::remove_pointer<T>::type,
-                                                       typename std::remove_pointer<U>::type>::value))>::type>
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
   MOZ_IMPLICIT
   Maybe(const Maybe<U>& aOther)
     : mIsSome(false)
@@ -131,16 +126,11 @@ public:
   }
 
   /**
-   * Maybe<T*> can be move-constructed from a Maybe<U*> if U* and T* are
-   * compatible, or from Maybe<decltype(nullptr)>.
+   * Maybe<T> can be move-constructed from a Maybe<U> if U is convertible to T.
    */
   template<typename U,
            typename =
-             typename std::enable_if<std::is_pointer<T>::value &&
-                                     (std::is_same<U, decltype(nullptr)>::value ||
-                                      (std::is_pointer<U>::value &&
-                                       std::is_base_of<typename std::remove_pointer<T>::type,
-                                                       typename std::remove_pointer<U>::type>::value))>::type>
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
   MOZ_IMPLICIT
   Maybe(Maybe<U>&& aOther)
     : mIsSome(false)
@@ -156,13 +146,7 @@ public:
     if (&aOther != this) {
       if (aOther.mIsSome) {
         if (mIsSome) {
-          // XXX(seth): The correct code for this branch, below, can't be used
-          // due to a bug in Visual Studio 2010. See bug 1052940.
-          /*
           ref() = aOther.ref();
-          */
-          reset();
-          emplace(*aOther);
         } else {
           emplace(*aOther);
         }
@@ -173,6 +157,23 @@ public:
     return *this;
   }
 
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
+  Maybe& operator=(const Maybe<U>& aOther)
+  {
+    if (aOther.isSome()) {
+      if (mIsSome) {
+        ref() = aOther.ref();
+      } else {
+        emplace(*aOther);
+      }
+    } else {
+      reset();
+    }
+    return *this;
+  }
+
   Maybe& operator=(Maybe&& aOther)
   {
     MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
@@ -191,6 +192,25 @@ public:
     return *this;
   }
 
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_convertible<U, T>::value>::type>
+  Maybe& operator=(Maybe<U>&& aOther)
+  {
+    if (aOther.isSome()) {
+      if (mIsSome) {
+        ref() = Move(aOther.ref());
+      } else {
+        emplace(Move(*aOther));
+      }
+      aOther.reset();
+    } else {
+      reset();
+    }
+
+    return *this;
+  }
+
   /* Methods that check whether this Maybe contains a value */
   explicit operator bool() const { return isSome(); }
   bool isSome() const { return mIsSome; }
@@ -443,11 +463,12 @@ public:
  * if you need to construct a Maybe value that holds a const, volatile, or
  * reference value, you need to use emplace() instead.
  */
-template<typename T>
-Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
+template<typename T,
+         typename U = typename std::remove_cv<
+           typename std::remove_reference<T>::type>::type>
+Maybe<U>
 Some(T&& aValue)
 {
-  typedef typename RemoveCV<typename RemoveReference<T>::Type>::Type U;
   Maybe<U> value;
   value.emplace(Forward<T>(aValue));
   return value;
diff --git a/netwerk/protocol/http/nsCORSListenerProxy.cpp b/netwerk/protocol/http/nsCORSListenerProxy.cpp
index 499c03094b..d2c37415e5 100644
--- a/netwerk/protocol/http/nsCORSListenerProxy.cpp
+++ b/netwerk/protocol/http/nsCORSListenerProxy.cpp
@@ -1344,7 +1344,7 @@ nsCORSPreflightListener::CheckPreflightRequestApproved(nsIRequest* aRequest)
                           headerVal);
   nsTArray<nsCString> headers;
   nsCCharSeparatedTokenizer headerTokens(headerVal, ',');
-  bool allowAllHeaders = false;
+  bool wildcard = false;
   while(headerTokens.hasMoreTokens()) {
     const nsDependentCSubstring& header = headerTokens.nextToken();
     if (header.IsEmpty()) {
@@ -1356,19 +1356,31 @@ nsCORSPreflightListener::CheckPreflightRequestApproved(nsIRequest* aRequest)
       return NS_ERROR_DOM_BAD_URI;
     }
     if (header.EqualsLiteral("*") && !mWithCredentials) {
-      allowAllHeaders = true;
+      wildcard = true;
     } else {
       headers.AppendElement(header);
     }
   }
-  if (!allowAllHeaders) {
-    for (uint32_t i = 0; i < mPreflightHeaders.Length(); ++i) {
-      if (!headers.Contains(mPreflightHeaders[i],
-                            nsCaseInsensitiveCStringArrayComparator())) {
-        LogBlockedRequest(aRequest, "CORSMissingAllowHeaderFromPreflight",
-                          NS_ConvertUTF8toUTF16(mPreflightHeaders[i]).get());
-        return NS_ERROR_DOM_BAD_URI;
-      }
+  for (uint32_t i = 0; i < mPreflightHeaders.Length(); ++i) {
+    if (wildcard
+        // Access-Control-Allow-Headers is '*', so we should skip these checks.
+#if 0
+        && !mPreflightHeaders[i].LowerCaseEqualsASCII("authorization")
+        // However, according to the spec, 'Authorization' isn't allowed to be
+        // wildcarded here and must always be explicitly mentioned.
+        // Fixme: Mainstream keeps this disabled because nobody obeys this rule.
+        //        This should be flipped on when either mainstream does or when there's enough
+        //        effort to make websites adhere to the spec, to keep our implementation
+        //        in line with the consensus on the web.
+#endif
+        ) {
+      continue;
+    }
+    if (!headers.Contains(mPreflightHeaders[i],
+                          nsCaseInsensitiveCStringArrayComparator())) {
+      LogBlockedRequest(aRequest, "CORSMissingAllowHeaderFromPreflight",
+                        NS_ConvertUTF8toUTF16(mPreflightHeaders[i]).get());
+      return NS_ERROR_DOM_BAD_URI;
     }
   }
 
diff --git a/xpcom/io/nsLocalFileWin.cpp b/xpcom/io/nsLocalFileWin.cpp
index afa055d9c5..5182b71f6f 100644
--- a/xpcom/io/nsLocalFileWin.cpp
+++ b/xpcom/io/nsLocalFileWin.cpp
@@ -3028,6 +3028,8 @@ nsLocalFile::IsExecutable(bool* aResult)
       "com",
       "cpl",
       "crt",
+      "der",
+      "diagcab",
       "exe",
       "fxp",         // FoxPro compiled app
       "hlp",