diff --git a/browser/base/content/browser-context.inc b/browser/base/content/browser-context.inc
index 526f0f69a6..bbdb0139fd 100644
--- a/browser/base/content/browser-context.inc
+++ b/browser/base/content/browser-context.inc
@@ -159,6 +159,9 @@
                 label="&mediaHideControls.label;"
                 accesskey="&mediaHideControls.accesskey;"
                 oncommand="gContextMenu.mediaCommand('hidecontrols');"/>
+      <menuitem id="context-video-pictureinpicture"
+                label="&pictureInPicture.title;"
+                oncommand="gContextMenu.pictureInPicture();"/>
       <menuitem id="context-video-fullscreen"
                 accesskey="&videoFullScreen.accesskey;"
                 label="&videoFullScreen.label;"
diff --git a/browser/base/content/content.js b/browser/base/content/content.js
index 0ddb15988d..4b5bb9ac4c 100644
--- a/browser/base/content/content.js
+++ b/browser/base/content/content.js
@@ -681,6 +681,17 @@ addMessageListener("ContextMenu:MediaCommand", (message) => {
     () => {
       let media = message.objects.element;
       switch (message.data.command) {
+        case "pictureinpicture": {
+          let {PictureInPicture} = Cu.import("resource:///modules/PictureInPicture.jsm", {});
+          let data;
+          try {
+            data = PictureInPicture.register(media, message.data.data);
+          } catch (error) {
+            data = {id: message.data.data, error: error.message};
+          }
+          sendAsyncMessage("PictureInPicture:Prepared", data);
+          break;
+        }
         case "play":
           media.play();
           break;
diff --git a/browser/base/content/nsContextMenu.js b/browser/base/content/nsContextMenu.js
index 5a47c1269f..61740a1599 100644
--- a/browser/base/content/nsContextMenu.js
+++ b/browser/base/content/nsContextMenu.js
@@ -442,6 +442,11 @@ nsContextMenu.prototype = {
     this.showItem("context-media-loop", onMedia);
     this.showItem("context-media-showcontrols", onMedia && !this.target.controls);
     this.showItem("context-media-hidecontrols", this.target.controls && (this.onVideo || (this.onAudio && !this.inSyntheticDoc)));
+    this.showItem("context-video-pictureinpicture", this.onVideo);
+    this.setItemAttr("context-video-pictureinpicture", "disabled",
+                     !this.onVideo || this.target.error ||
+                     !this.target.videoWidth || !this.target.videoHeight ||
+                     !!this.target.mediaKeys);
     this.showItem("context-video-fullscreen", this.onVideo && this.target.ownerDocument.fullscreenElement == null);
     this.showItem("context-media-eme-learnmore", this.onDRMMedia);
     this.showItem("context-media-eme-separator", this.onDRMMedia);
@@ -1682,6 +1687,11 @@ nsContextMenu.prototype = {
     this.browser.messageManager.sendAsyncMessage("SwitchDocumentDirection");
   },
 
+  pictureInPicture: function() {
+    let {PictureInPicture} = Cu.import("resource:///modules/PictureInPicture.jsm", {});
+    PictureInPicture.open(this.browser, this.target);
+  },
+
   mediaCommand : function(command, data) {
     let mm = this.browser.messageManager;
     let win = this.browser.ownerGlobal;
diff --git a/browser/base/content/pictureInPicture.js b/browser/base/content/pictureInPicture.js
new file mode 100644
index 0000000000..ce9cc5a894
--- /dev/null
+++ b/browser/base/content/pictureInPicture.js
@@ -0,0 +1,87 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var Player = {
+  browser: null,
+  source: null,
+  sourceTab: null,
+
+  init() {
+    let args = window.arguments[0];
+    this.source = args.browser;
+    if (!this.source || !this.source.isConnected) {
+      window.close();
+      return;
+    }
+    this.sourceTab = this.source.ownerGlobal.gBrowser.getTabForBrowser(this.source);
+    this.close = () => window.close();
+    this.sourceTab.addEventListener("TabClose", this.close);
+    this.source.addEventListener("oop-browser-crashed", this.close);
+
+    this.playLabel = document.getElementById("play").label;
+    this.muteLabel = document.getElementById("mute").label;
+    let browser = this.browser = document.createElement("browser");
+    browser.setAttribute("type", "content");
+    browser.setAttribute("flex", "1");
+    browser.setAttribute("disablehistory", "true");
+    browser.setAttribute("remote", this.source.isRemoteBrowser ? "true" : "false");
+    if (this.source.isRemoteBrowser) {
+      browser.setAttribute("remoteType", this.source.getAttribute("remoteType"));
+    }
+    // The process-local source reference requires this process affinity.
+    browser.relatedBrowser = this.source;
+    document.getElementById("player-container").appendChild(browser);
+    let mm = browser.messageManager;
+    mm.addMessageListener("PictureInPicture:Ready", () => {
+      mm.sendAsyncMessage("PictureInPicture:Init", {id: args.id});
+    });
+    mm.addMessageListener("PictureInPicture:Close", this.close);
+    mm.addMessageListener("PictureInPicture:State", message => {
+      document.getElementById("play").label = message.data.paused ?
+        this.playLabel : document.getElementById("pause-label").value;
+      document.getElementById("mute").label = message.data.muted ?
+        document.getElementById("unmute-label").value : this.muteLabel;
+    });
+    browser.addEventListener("oop-browser-crashed", this.close);
+    mm.loadFrameScript("chrome://browser/content/pictureInPictureContent.js", false);
+  },
+
+  command(command) {
+    if (this.browser) {
+      this.browser.messageManager.sendAsyncMessage("PictureInPicture:Command", {command});
+    }
+  },
+
+  returnToTab() {
+    if (this.source && this.source.isConnected) {
+      let win = this.source.ownerGlobal;
+      win.gBrowser.selectedTab = this.sourceTab;
+      win.focus();
+    }
+    window.close();
+  },
+
+  destroy() {
+    if (this.sourceTab) {
+      this.sourceTab.removeEventListener("TabClose", this.close);
+      this.source.removeEventListener("oop-browser-crashed", this.close);
+    }
+    if (this.browser) {
+      this.command("close");
+    }
+    this.source = this.sourceTab = null;
+  },
+};
+
+window.addEventListener("load", () => Player.init(), {once: true});
+window.addEventListener("unload", () => Player.destroy(), {once: true});
+window.addEventListener("keydown", event => {
+  if (event.key == "Escape") {
+    window.close();
+  } else if (event.key == " " && event.target.localName != "button") {
+    event.preventDefault();
+    Player.command("playpause");
+  }
+});
diff --git a/browser/base/content/pictureInPicture.xul b/browser/base/content/pictureInPicture.xul
new file mode 100644
index 0000000000..c8d23854e9
--- /dev/null
+++ b/browser/base/content/pictureInPicture.xul
@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
+<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
+<!DOCTYPE window SYSTEM "chrome://browser/locale/browser.dtd">
+<window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        id="picture-in-picture" windowtype="Browser:PictureInPicture"
+        title="&pictureInPicture.title;" width="480" height="318"
+        persist="screenX screenY width height" style="min-width: 240px; min-height: 160px;">
+  <script type="application/javascript" src="chrome://browser/content/pictureInPicture.js"/>
+  <vbox id="player-container" flex="1" style="background: black;"/>
+  <hbox align="center" pack="center">
+    <button id="play" label="&mediaPlay.label;" oncommand="Player.command('playpause');"/>
+    <button id="mute" label="&mediaMute.label;" oncommand="Player.command('mute');"/>
+    <button id="return" label="&pictureInPicture.return;" oncommand="Player.returnToTab();"/>
+    <button id="close" label="&pictureInPicture.close;" oncommand="window.close();"/>
+  </hbox>
+  <label id="pause-label" value="&mediaPause.label;" hidden="true"/>
+  <label id="unmute-label" value="&mediaUnmute.label;" hidden="true"/>
+</window>
diff --git a/browser/base/content/pictureInPictureContent.js b/browser/base/content/pictureInPictureContent.js
new file mode 100644
index 0000000000..d665ecd10e
--- /dev/null
+++ b/browser/base/content/pictureInPictureContent.js
@@ -0,0 +1,129 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+var {PictureInPicture} = Components.utils.import(
+  "resource:///modules/PictureInPicture.jsm", {});
+var source = null;
+var stream = null;
+var output = null;
+var still = null;
+var observer = null;
+var sourceWindow = null;
+var events = ["play", "pause", "volumechange", "ended", "seeked", "resize"];
+
+function update() {
+  if (!source) {
+    return;
+  }
+  // Capture streams do not produce frames while their source is paused.
+  // Keep a snapshot for that case, without changing the source's play state.
+  still.hidden = !source.paused && !source.ended;
+  if (!still.hidden && source.readyState >= 2) {
+    still.width = source.videoWidth;
+    still.height = source.videoHeight;
+    still.getContext("2d").drawImage(source, 0, 0, still.width, still.height);
+  }
+  sendAsyncMessage("PictureInPicture:State",
+                   {paused: source.paused || source.ended, muted: source.muted});
+}
+
+function cleanup() {
+  if (observer) {
+    observer.disconnect();
+    observer = null;
+  }
+  if (source) {
+    for (let name of events) {
+      source.removeEventListener(name, update);
+    }
+    source.removeEventListener("emptied", closePlayer);
+    source.removeEventListener("error", closePlayer);
+    sourceWindow.removeEventListener("pagehide", closePlayer);
+    source.mozPictureInPicture = false;
+    source = sourceWindow = null;
+  }
+  if (output) {
+    output.srcObject = null;
+    output = null;
+  }
+  if (stream) {
+    for (let track of stream.getTracks()) {
+      track.stop();
+    }
+    stream = null;
+  }
+  still = null;
+}
+
+function closePlayer() {
+  cleanup();
+  sendAsyncMessage("PictureInPicture:Close");
+}
+
+addMessageListener("PictureInPicture:Init", message => {
+  try {
+    source = PictureInPicture.takeSource(message.data.id);
+    sourceWindow = source.ownerGlobal;
+    let doc = content.document;
+    doc.documentElement.style.cssText = "height:100%;background:black";
+    doc.body.style.cssText = "margin:0;height:100%;overflow:hidden";
+    output = doc.createElement("video");
+    still = doc.createElement("canvas");
+    for (let element of [output, still]) {
+      element.style.cssText = "position:absolute;width:100%;height:100%;object-fit:contain";
+      doc.body.appendChild(element);
+    }
+    output.muted = true; // Audio continues to come from the originating video.
+    source.mozPictureInPicture = true;
+    stream = source.mozCaptureStream();
+    output.srcObject = stream;
+    output.play().catch(error => {
+      Components.utils.reportError(error);
+      closePlayer();
+    });
+    for (let name of events) {
+      source.addEventListener(name, update);
+    }
+    source.addEventListener("emptied", closePlayer);
+    source.addEventListener("error", closePlayer);
+    sourceWindow.addEventListener("pagehide", closePlayer);
+    observer = new sourceWindow.MutationObserver(() => {
+      if (source && !source.isConnected) {
+        closePlayer();
+      }
+    });
+    observer.observe(source.ownerDocument, {childList: true, subtree: true});
+    update();
+  } catch (error) {
+    Components.utils.reportError(error);
+    closePlayer();
+  }
+});
+
+addMessageListener("PictureInPicture:Command", message => {
+  if (!source) {
+    return;
+  }
+  switch (message.data.command) {
+    case "playpause":
+      if (source.paused || source.ended) {
+        source.play().catch(Components.utils.reportError);
+      } else {
+        source.pause();
+      }
+      break;
+    case "mute":
+      source.muted = !source.muted;
+      break;
+    case "close":
+      cleanup();
+      break;
+  }
+});
+
+// Frame-script unload also covers a parent window closing before an async
+// close command can be delivered.
+addEventListener("unload", cleanup);
+sendAsyncMessage("PictureInPicture:Ready");
diff --git a/browser/base/jar.mn b/browser/base/jar.mn
index d31d47781f..9e1ab30e31 100644
--- a/browser/base/jar.mn
+++ b/browser/base/jar.mn
@@ -2,6 +2,9 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 browser.jar:
+        content/browser/pictureInPicture.xul          (content/pictureInPicture.xul)
+        content/browser/pictureInPicture.js           (content/pictureInPicture.js)
+        content/browser/pictureInPictureContent.js    (content/pictureInPictureContent.js)
 %  content browser %content/browser/ contentaccessible=yes
 #ifdef XP_MACOSX
 %  overlay chrome://mozapps/content/downloads/downloads.xul chrome://browser/content/downloadManagerOverlay.xul
diff --git a/browser/locales/en-US/chrome/browser/browser.dtd b/browser/locales/en-US/chrome/browser/browser.dtd
index 0d4f2c9503..814bcf288b 100644
--- a/browser/locales/en-US/chrome/browser/browser.dtd
+++ b/browser/locales/en-US/chrome/browser/browser.dtd
@@ -777,3 +777,7 @@ you can use these alternative items. Otherwise, their values should be empty.  -
 
 <!ENTITY emeLearnMoreContextMenu.label            "Learn more about DRM…">
 <!ENTITY emeLearnMoreContextMenu.accesskey        "D">
+
+<!ENTITY pictureInPicture.title "Picture-in-Picture">
+<!ENTITY pictureInPicture.return "Return to tab">
+<!ENTITY pictureInPicture.close "Close">
diff --git a/browser/modules/PictureInPicture.jsm b/browser/modules/PictureInPicture.jsm
new file mode 100644
index 0000000000..6547345e17
--- /dev/null
+++ b/browser/modules/PictureInPicture.jsm
@@ -0,0 +1,84 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["PictureInPicture"];
+
+const {utils: Cu} = Components;
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/Timer.jsm");
+
+// Each content process has its own module instance. Only a weak reference to
+// the source crosses between frame scripts; DOM objects never cross processes.
+this.PictureInPicture = {
+  source: null,
+  window: null,
+  request: 0,
+
+  register(video, id) {
+    if (!video || video.localName != "video" || video.error ||
+        !video.videoWidth || !video.videoHeight || video.mediaKeys) {
+      throw new Error("This video is not available for Picture-in-Picture");
+    }
+    this.source = {id, video: Cu.getWeakReference(video)};
+    return {id, width: video.videoWidth, height: video.videoHeight};
+  },
+
+  takeSource(id) {
+    if (!this.source || this.source.id != id) {
+      throw new Error("Picture-in-Picture source process is unavailable");
+    }
+    let video = this.source.video.get();
+    this.source = null;
+    if (!video || !video.isConnected) {
+      throw new Error("Picture-in-Picture source has been removed");
+    }
+    return video;
+  },
+
+  open(browser, video) {
+    let id = Services.uuid.generateUUID().toString();
+    let request = ++this.request;
+    let mm = browser.messageManager;
+    let timer;
+    let ready = message => {
+      if (message.data.id != id) {
+        return;
+      }
+      mm.removeMessageListener("PictureInPicture:Prepared", ready);
+      clearTimeout(timer);
+      if (request != this.request || !browser.isConnected) {
+        return;
+      }
+      if (message.data.error) {
+        Cu.reportError(message.data.error);
+        return;
+      }
+      this.openWindow(browser, message.data);
+    };
+    mm.addMessageListener("PictureInPicture:Prepared", ready);
+    timer = setTimeout(() => {
+      mm.removeMessageListener("PictureInPicture:Prepared", ready);
+    }, 10000);
+    mm.sendAsyncMessage("ContextMenu:MediaCommand",
+                        {command: "pictureinpicture", data: id},
+                        {element: video});
+  },
+
+  openWindow(browser, data) {
+    if (this.window && !this.window.closed) {
+      this.window.close();
+    }
+    let screen = browser.ownerGlobal.screen;
+    let width = Math.min(480, screen.availWidth);
+    let height = Math.min(Math.round(width * data.height / data.width) + 48,
+                          Math.round(screen.availHeight * 0.8));
+    this.window = browser.ownerGlobal.openDialog(
+      "chrome://browser/content/pictureInPicture.xul", "",
+      "chrome,dialog=no,resizable,alwaysRaised,width=" + width +
+      ",height=" + height,
+      {browser, id: data.id});
+    return this.window;
+  },
+};
diff --git a/browser/modules/moz.build b/browser/modules/moz.build
index 881ff98f36..bcd0e59190 100644
--- a/browser/modules/moz.build
+++ b/browser/modules/moz.build
@@ -22,6 +22,7 @@ EXTRA_JS_MODULES += [
     'NetworkPrioritizer.jsm',
     'offlineAppCache.jsm',
     'PermissionUI.jsm',
+    'PictureInPicture.jsm',
     'PluginContent.jsm',
     'ProcessHangMonitor.jsm',
     'ReaderParent.jsm',
diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index 2f2d7a1a7a..2edcfc93da 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -5183,6 +5183,15 @@ bool HTMLMediaElement::IsActive() const
   return ownerDoc && ownerDoc->IsActive() && ownerDoc->IsVisible();
 }
 
+void HTMLMediaElement::SetMozPictureInPicture(bool aEnabled)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  mPictureInPicture = aEnabled;
+  if (mDecoder && !mDecoder->IsShutdown()) {
+    mDecoder->NotifyOwnerActivityChanged(!IsHidden());
+  }
+}
+
 bool HTMLMediaElement::IsHidden() const
 {
   nsIDocument* ownerDoc;
diff --git a/dom/html/HTMLMediaElement.h b/dom/html/HTMLMediaElement.h
index 5943c08419..11cf27ba30 100644
--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -211,6 +211,9 @@ public:
 
   virtual bool IsHidden() const final override;
 
+  bool MozPictureInPicture() const { return mPictureInPicture; }
+  void SetMozPictureInPicture(bool aEnabled);
+
   // Called by the media decoder and the video frame to get the
   // ImageContainer containing the video data.
   virtual VideoFrameContainer* GetVideoFrameContainer() final override;
@@ -1622,6 +1625,9 @@ protected:
   // Info about the played media.
   MediaInfo mMediaInfo;
 
+  // Set only by privileged Picture-in-Picture UI; cleared when it closes.
+  bool mPictureInPicture = false;
+
   // True if the media has encryption information.
   bool mIsEncrypted;
 
diff --git a/dom/media/MediaDecoder.cpp b/dom/media/MediaDecoder.cpp
index 2e23dec1e6..1130d0a0ce 100644
--- a/dom/media/MediaDecoder.cpp
+++ b/dom/media/MediaDecoder.cpp
@@ -1250,7 +1250,11 @@ MediaDecoder::SetElementVisibility(bool aIsVisible)
 {
   MOZ_ASSERT(NS_IsMainThread());
   mElementVisible = aIsVisible;
-  mIsVisible = !mForcedHidden && mElementVisible;
+  // A floating player remains visible when its source tab is in the
+  // background or the original video is scrolled out of view.
+  HTMLMediaElement* element = mOwner ? mOwner->GetMediaElement() : nullptr;
+  mIsVisible = (element && element->MozPictureInPicture()) ||
+               (!mForcedHidden && mElementVisible);
 }
 
 void
diff --git a/dom/webidl/HTMLVideoElement.webidl b/dom/webidl/HTMLVideoElement.webidl
index 5ae4c016f1..39144039f7 100644
--- a/dom/webidl/HTMLVideoElement.webidl
+++ b/dom/webidl/HTMLVideoElement.webidl
@@ -46,6 +46,10 @@ partial interface HTMLVideoElement {
   // True if the video has an audio track available.
   readonly attribute boolean mozHasAudio;
 
+  // Browser UI only: keep the source decoder active for a floating player.
+  [ChromeOnly]
+  attribute boolean mozPictureInPicture;
+
   // True if the video should use a screen wake lock.
   [Pref="dom.wakelock.enabled", Func="Navigator::HasWakeLockSupport"]
   attribute boolean mozUseScreenWakeLock;