diff --git a/image/imgLoader.cpp b/image/imgLoader.cpp
index d8e50cb2eb..f3e614d0a2 100644
--- a/image/imgLoader.cpp
+++ b/image/imgLoader.cpp
@@ -2489,6 +2489,13 @@ imgLoader::GetMIMETypeFromContent(nsIRequest* aRequest,
                                   uint32_t aLength,
                                   nsACString& aContentType)
 {
+  nsCOMPtr<nsIChannel> channel(do_QueryInterface(aRequest));
+  if (channel) {
+    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+    if (loadInfo->GetSkipContentSniffing()) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+  }
   return GetMimeTypeFromContent((const char*)aContents, aLength, aContentType);
 }
 
diff --git a/netwerk/base/nsNetUtil.cpp b/netwerk/base/nsNetUtil.cpp
index 2207e9e24d..ce62cfa8fc 100644
--- a/netwerk/base/nsNetUtil.cpp
+++ b/netwerk/base/nsNetUtil.cpp
@@ -16,7 +16,6 @@
 #include "nsContentUtils.h"
 #include "nsHashKeys.h"
 #include "nsHttp.h"
-#include "nsMimeTypes.h"
 #include "nsIAsyncStreamCopier.h"
 #include "nsIAuthPrompt.h"
 #include "nsIAuthPrompt2.h"
@@ -2150,33 +2149,6 @@ NS_SniffContent(const char *aSnifferType, nsIRequest *aRequest,
     return;
   }
 
-  aSniffedType.Truncate();
-
-  nsCOMPtr<nsIChannel> channel = do_QueryInterface(aRequest);
-  if (channel) {
-    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
-    if (loadInfo->GetSkipContentSniffing()) {
-      // In case XCTO nosniff was present, we should skip sniffing here, but...
-      nsAutoCString currentContentType;
-      channel->GetContentType(currentContentType);
-      // We cannot skip sniffing if the current MIME type is a JSON file.
-      // The JSON-Viewer relies on its own sniffer to determine if it can render
-      // the page, so we need to make an exception if the Server provides a valid
-      // JSON MIME type (application/json, application/web-manifest or text/json).
-      // We also don't skip sniffing if the currently-known content type is empty,
-      // to deal with webmaster errors (nosniff is set but no content-type supplied)
-      // See Issue #2258.
-      if (!currentContentType.Equals(APPLICATION_JSON) && 
-          !currentContentType.Equals(APPLICATION_WEB_MANIFEST) &&
-          !currentContentType.Equals(TEXT_JSON) &&
-          !currentContentType.IsEmpty()) {
-        // Content type supplied and it's not a JSON type; honor XCTO:nosniff.
-        return;
-      }
-    }
-  }
-  
-  // Iterate through the sniffers...
   nsCOMArray<nsIContentSniffer> sniffers;
   cache->GetEntries(sniffers);
   for (int32_t i = 0; i < sniffers.Count(); ++i) {
@@ -2185,9 +2157,8 @@ NS_SniffContent(const char *aSnifferType, nsIRequest *aRequest,
       return;
     }
   }
-  
-  // If we get here, there's nothing more to be done, return with a truncated aSniffedType.
 
+  aSniffedType.Truncate();
 }
 
 bool
diff --git a/netwerk/protocol/http/nsHttpChannel.cpp b/netwerk/protocol/http/nsHttpChannel.cpp
index c28606982e..dba009ea30 100644
--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -1106,7 +1106,7 @@ ProcessXCTO(nsIURI* aURI, nsHttpResponseHead* aResponseHead, nsILoadInfo* aLoadI
         policyType == nsIContentPolicy::TYPE_SUBDOCUMENT) {
       // If the header XCTO nosniff is set for any browsing context, then
       // we set the skipContentSniffing flag on the Loadinfo. Within
-      // NS_SniffContent we then bail early and do not do any sniffing.
+      // GetMIMETypeFromContent we then bail early and do not do any sniffing.
       aLoadInfo->SetSkipContentSniffing(true);
       return NS_OK;
     }
diff --git a/netwerk/streamconv/converters/nsUnknownDecoder.cpp b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
index d4aa09f2b9..bab9b84c98 100644
--- a/netwerk/streamconv/converters/nsUnknownDecoder.cpp
+++ b/netwerk/streamconv/converters/nsUnknownDecoder.cpp
@@ -316,6 +316,13 @@ nsUnknownDecoder::GetMIMETypeFromContent(nsIRequest* aRequest,
                                          nsACString& type)
 {
   // Note: This is only used by sniffer, therefore we do not need to lock anything here.
+  nsCOMPtr<nsIChannel> channel(do_QueryInterface(aRequest));
+  if (channel) {
+    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+    if (loadInfo->GetSkipContentSniffing()) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
+  }
   mBuffer = const_cast<char*>(reinterpret_cast<const char*>(aData));
   mBufferLen = aLength;
   DetermineContentType(aRequest);
@@ -346,6 +353,11 @@ bool nsUnknownDecoder::AllowSniffing(nsIRequest* aRequest)
     return false;
   }
   
+  nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+  if (loadInfo->GetSkipContentSniffing()) {
+    return false;
+  }
+  
   bool isLocalFile = false;
   if (NS_FAILED(uri->SchemeIs("file", &isLocalFile)) || isLocalFile) {
     return false;
@@ -393,6 +405,13 @@ void nsUnknownDecoder::DetermineContentType(nsIRequest* aRequest)
   if (!mContentType.IsEmpty()) return;
 
   nsCOMPtr<nsIHttpChannel> channel(do_QueryInterface(aRequest));
+  if (channel) {
+    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+    if (loadInfo->GetSkipContentSniffing()) {
+      return;
+    }
+  }
+
   const char* testData = mBuffer;
   uint32_t testDataLen = mBufferLen;
   // Check if data are compressed.
@@ -564,6 +583,9 @@ bool nsUnknownDecoder::SniffURI(nsIRequest* aRequest)
 {
   nsCOMPtr<nsIChannel> channel(do_QueryInterface(aRequest));
   nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+  if (loadInfo->GetSkipContentSniffing()) {
+    return false;
+  }
   nsCOMPtr<nsIMIMEService> mimeService(do_GetService("@mozilla.org/mime;1"));
   if (mimeService) {
     nsCOMPtr<nsIChannel> channel = do_QueryInterface(aRequest);
diff --git a/toolkit/components/mediasniffer/nsMediaSniffer.cpp b/toolkit/components/mediasniffer/nsMediaSniffer.cpp
index d00dea894b..5c6fee065e 100644
--- a/toolkit/components/mediasniffer/nsMediaSniffer.cpp
+++ b/toolkit/components/mediasniffer/nsMediaSniffer.cpp
@@ -138,6 +138,10 @@ nsMediaSniffer::GetMIMETypeFromContent(nsIRequest* aRequest,
                                        nsACString& aSniffedType) {
   nsCOMPtr<nsIChannel> channel = do_QueryInterface(aRequest);
   if (channel) {
+    nsCOMPtr<nsILoadInfo> loadInfo = channel->GetLoadInfo();
+    if (loadInfo->GetSkipContentSniffing()) {
+      return NS_ERROR_NOT_AVAILABLE;
+    }
     nsLoadFlags loadFlags = 0;
     channel->GetLoadFlags(&loadFlags);
     if (!(loadFlags & nsIChannel::LOAD_MEDIA_SNIFFER_OVERRIDES_CONTENT_TYPE)) {