ported from UXP:

- Allow matroska and avc (h.264) content in matroska/webm containers for video element and MSE. (18f9b185)
- Alow AAC audio codec data in matroska/webm streams. Allow CRC32 elements in matroska cluster elements. (6b6aa59f)
- Truncate Buffer/Texture on GL_OOM. (3f9e299b)
- Bug 1550498 (b0a8ed21)
- Don't allow cross-origin POST redirects on 308 codes. (8b7daa53)
- Bug 1548822 (b7807cd0)
- Do not allow the ^ character to appear in the hostname. (f046b917)
- Add CheckedInt check for GL texture uploads. (227b2360)
- Implement a threadsafe & revised version of http2PushedStream. (17a4b4f6d)
- Convert dom/base/nsImageLoadingContent.cpp to use AsyncOpen2 and followups along with it (1445670 and 1373780 part 2 and 3) (deae241f)
and ported part of tenfourfox changes:
- #559: M1550498 M1548822 M1540759(partial) M1528481(+WeakPtr for Http2Stream) M1555523 M1552541 (46496870)
- #559: M1547266 (97b8db0b)

parser/html/nsHtml5TreeOpExecutor.cpp

@@ -956,8 +956,9 @@ nsHtml5TreeOpExecutor::PreloadImage(const nsAString& aURL,
                                     const nsAString& aImageReferrerPolicy)
 {
   nsCOMPtr<nsIURI> baseURI = BaseURIForPreload();
+  bool isImgSet = false;
   nsCOMPtr<nsIURI> uri = mDocument->ResolvePreloadImage(baseURI, aURL, aSrcset,
-                                                        aSizes);
+                                                        aSizes, &isImgSet);
   if (uri && ShouldPreloadURI(uri)) {
     // use document wide referrer policy
     mozilla::net::ReferrerPolicy referrerPolicy = mSpeculationReferrerPolicy;
@@ -971,7 +972,7 @@ nsHtml5TreeOpExecutor::PreloadImage(const nsAString& aURL,
       }
     }
 
-    mDocument->MaybePreLoadImage(uri, aCrossOrigin, referrerPolicy);
+    mDocument->MaybePreLoadImage(uri, aCrossOrigin, referrerPolicy, isImgSet);
   }
 }