diff --git a/dom/base/nsObjectLoadingContent.cpp b/dom/base/nsObjectLoadingContent.cpp
index 2c5538998b57..66315f40dcfd 100644
--- a/dom/base/nsObjectLoadingContent.cpp
+++ b/dom/base/nsObjectLoadingContent.cpp
@@ -103,6 +103,7 @@
 static NS_DEFINE_CID(kAppShellCID, NS_APPSHELL_CID);
 
 static const char *kPrefJavaMIME = "plugin.java.mime";
+static const char *kPrefYoutubeRewrite = "plugins.rewrite_youtube_embeds";
 
 using namespace mozilla;
 using namespace mozilla::dom;
@@ -714,7 +715,8 @@ nsObjectLoadingContent::nsObjectLoadingContent()
   , mActivated(false)
   , mIsStopping(false)
   , mIsLoading(false)
-  , mScriptRequested(false) {}
+  , mScriptRequested(false)
+  , mRewrittenYoutubeEmbed(false) {}
 
 nsObjectLoadingContent::~nsObjectLoadingContent()
 {
@@ -1482,7 +1484,7 @@ nsObjectLoadingContent::CheckJavaCodebase()
 }
 
 bool
-nsObjectLoadingContent::IsYoutubeEmbed()
+nsObjectLoadingContent::ShouldRewriteYoutubeEmbed(nsIURI* aURI)
 {
   nsCOMPtr<nsIContent> thisContent =
     do_QueryInterface(static_cast<nsIImageLoadingContent*>(this));
@@ -1500,25 +1502,41 @@ nsObjectLoadingContent::IsYoutubeEmbed()
     NS_WARNING("Could not get TLD service!");
     return false;
   }
+
   nsAutoCString currentBaseDomain;
-  bool ok = NS_SUCCEEDED(tldService->GetBaseDomain(mURI, 0, currentBaseDomain));
+  bool ok = NS_SUCCEEDED(tldService->GetBaseDomain(aURI, 0, currentBaseDomain));
   if (!ok) {
-    // Data URIs won't parse correctly, so just fail silently here.
+    // Data URIs (commonly used for things like svg embeds) won't parse
+    // correctly, so just fail silently here.
     return false;
   }
+
   // See if URL is referencing youtube
-  nsAutoCString domain("youtube.com");
-  if (!StringEndsWith(domain, currentBaseDomain)) {
+  if (!currentBaseDomain.EqualsLiteral("youtube.com")) {
     return false;
   }
+
+  // We should only rewrite URLs with paths starting with "/v/", as we shouldn't
+  // touch object nodes with "/embed/" urls that already do that right thing.
+  nsAutoCString path;
+  aURI->GetPath(path);
+  if (!StringBeginsWith(path, NS_LITERAL_CSTRING("/v/"))) {
+    return false;
+  }
+
   // See if requester is planning on using the JS API.
   nsAutoCString uri;
-  mURI->GetSpec(uri);
-  // Only log urls that are rewritable, e.g. not using enablejsapi=1
+  aURI->GetSpec(uri);
   if (uri.Find("enablejsapi=1", true, 0, -1) != kNotFound) {
     return false;
   }
-  return true;
+
+  // If we've made it this far, we've got a rewritable embed. Log it in
+  // telemetry.
+  Telemetry::Accumulate(Telemetry::YOUTUBE_REWRITABLE_EMBED_SEEN, 1);
+
+  // Even if node is rewritable, only rewrite if the pref tells us we should.
+  return Preferences::GetBool(kPrefYoutubeRewrite);
 }
 
 bool
@@ -1769,6 +1787,7 @@ nsObjectLoadingContent::UpdateObjectParameters(bool aJavaURI)
     NS_NOTREACHED("Unrecognized plugin-loading tag");
   }
 
+  mRewrittenYoutubeEmbed = false;
   // Note that the baseURI changing could affect the newURI, even if uriStr did
   // not change.
   if (!uriStr.IsEmpty()) {
@@ -1776,6 +1795,19 @@ nsObjectLoadingContent::UpdateObjectParameters(bool aJavaURI)
                                                    uriStr,
                                                    thisContent->OwnerDoc(),
                                                    newBaseURI);
+    if (ShouldRewriteYoutubeEmbed(newURI)) {
+      // Switch out video access url formats, which should possibly allow HTML5
+      // video loading.
+      uriStr.ReplaceSubstring(NS_LITERAL_STRING("/v/"),
+                              NS_LITERAL_STRING("/embed/"));
+      rv = nsContentUtils::NewURIWithDocumentCharset(getter_AddRefs(newURI),
+                                                     uriStr,
+                                                     thisContent->OwnerDoc(),
+                                                     newBaseURI);
+      mRewrittenYoutubeEmbed = true;
+      newMime = NS_LITERAL_CSTRING("text/html");
+    }
+
     if (NS_SUCCEEDED(rv)) {
       NS_TryToSetImmutable(newURI);
     } else {
@@ -1939,9 +1971,9 @@ nsObjectLoadingContent::UpdateObjectParameters(bool aJavaURI)
     newType = eType_Null;
     newMime.Truncate();
   } else if (newChannel) {
-      // If newChannel is set above, we considered it in setting newMime
-      newType = GetTypeOfContent(newMime);
-      LOG(("OBJLC [%p]: Using channel type", this));
+    // If newChannel is set above, we considered it in setting newMime
+    newType = GetTypeOfContent(newMime);
+    LOG(("OBJLC [%p]: Using channel type", this));
   } else if (((caps & eAllowPluginSkipChannel) || !newURI) &&
              GetTypeOfContent(newMime) == eType_Plugin) {
     newType = eType_Plugin;
@@ -2170,11 +2202,6 @@ nsObjectLoadingContent::LoadObject(bool aNotify,
     return NS_OK;
   }
 
-  // Check whether this is a youtube embed.
-  if (IsYoutubeEmbed()) {
-    Telemetry::Accumulate(Telemetry::YOUTUBE_REWRITABLE_EMBED_SEEN, 1);
-  }
-
   //
   // Security checks
   //
diff --git a/dom/base/nsObjectLoadingContent.h b/dom/base/nsObjectLoadingContent.h
index 0c317d222a5e..8d788c9846b9 100644
--- a/dom/base/nsObjectLoadingContent.h
+++ b/dom/base/nsObjectLoadingContent.h
@@ -331,6 +331,12 @@ class nsObjectLoadingContent : public nsImageLoadingContent
      */
     virtual nsContentPolicyType GetContentPolicyType() const = 0;
 
+    // True if object represents an object/embed tag pointing to a flash embed
+    // for a youtube video. When possible (see IsRewritableYoutubeEmbed function
+    // comments for details), we change these to try to load HTML5 versions of
+    // videos.
+    bool                        mRewrittenYoutubeEmbed : 1;
+
   private:
 
     // Object parameter changes returned by UpdateObjectParameters
@@ -519,7 +525,27 @@ class nsObjectLoadingContent : public nsImageLoadingContent
      */
     nsPluginFrame* GetExistingFrame();
 
-    bool IsYoutubeEmbed();
+    /**
+     * Used for identifying whether we can rewrite a youtube flash embed to
+     * possibly use HTML5 instead.
+     *
+     * Returns true if plugin.rewrite_youtube_embeds pref is true and the
+     * element this nsObjectLoadingContent instance represents:
+     *
+     * - is an embed or object node
+     * - has a URL pointing at the youtube.com domain, using "/v/" style video
+     *   path reference, and without enablejsapi=1 in the path
+     *
+     * Having the enablejsapi flag means the document that contains the element
+     * could possibly be manipulating the youtube video elsewhere on the page
+     * via javascript. We can't rewrite these kinds of elements without possibly
+     * breaking content, which we want to avoid.
+     *
+     * If we can rewrite the URL, we change the "/v/" to "/embed/", and change
+     * our type to eType_Document so that we render similarly to an iframe
+     * embed.
+     */
+    bool ShouldRewriteYoutubeEmbed(nsIURI* uri);
 
     // Helper class for SetupProtoChain
     class SetupProtoChainRunner final : public nsIRunnable
diff --git a/dom/html/HTMLSharedObjectElement.cpp b/dom/html/HTMLSharedObjectElement.cpp
index cf954be6150e..4d6a29a3cfbf 100644
--- a/dom/html/HTMLSharedObjectElement.cpp
+++ b/dom/html/HTMLSharedObjectElement.cpp
@@ -345,6 +345,11 @@ HTMLSharedObjectElement::GetCapabilities() const
   if (mNodeInfo->Equals(nsGkAtoms::embed)) {
     capabilities |= eSupportSVG | eSupportImages;
   }
+  // If this is a rewritten youtube flash embed, add documents to capabilities
+  // so that we can render HTML5 if possible.
+  if (mRewrittenYoutubeEmbed) {
+    capabilities |= eSupportDocuments;
+  }
 
   return capabilities;
 }
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 7e7ac9b322d0..5b428d2c9dc5 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5140,3 +5140,7 @@ pref("webextensions.tests", false);
 
 // Allow customization of the fallback directory for file uploads
 pref("dom.input.fallbackUploadDir", "");
+
+// Turn rewriting of youtube embeds on/off
+pref("plugins.rewrite_youtube_embeds", true);
+