Speed & memory:
- Fixed: 784591 - Don't leave around decoded image data for display: none images.
- Fixed: 836010 - When startup is determined to be slow, tell users about ways to improve their startup time.
- Fixed: 823147 - Avoid hitting D2D slow path when drawing radial gradients from css.
- Fixed: 821361 - JS: Optimize closures that only run once.
- Fixed: 807853 - JS: Add parallel compilation mode.
- Fixed: 715419 - JS: Specialize Array.prototype.sort when given the comparator is "return arg1 - arg2".
- Fixed: 835417 - DOM: Mark DOM getters as pure when they are.
- Fixed: 239254 - [Linux] Support disk cache on a local path.
New web technologies:
- Fixed: 820508 - Add support for <main> element.
- Fixed: 508725 - Implement scoped stylesheets.
- Fixed: 440046 - Implement window.crypto.getRandomValues (secure PRNG).
- Fixed: 828261 - Implement window.location.origin.
- Fixed: 792263 - Implement decodeAudioData.
- Fixed: 796463 - Enable WebRTC's PeerConnection by default.
For more, read Firefox 21 for developers.
Security & privacy:
- Fixed: 838557 - Create and use a common interface for ASan/Valgrind memory handling functions.
- Fixed: 821892 - Click-to-play: "Page Info" -> Permissions needs to be aware of plugin permission differentiation.
- Fixed: 774315 - Click-to-play: Add "Hide this plugin" to context menu, to work around broken sites.
- Fixed: 822371 - Mixed content: Improved UI for users who have set security.mixed_content.block_active_content.
Other notable fixes:
- Fixed: 833511 - Add rotation gesture support to image documents.
- Fixed: 765398 - Implement three-state UI for DNT (do not track me, okay to track me, decline to state).
- Fixed: 815640 - Make permission manager aware of "file://" (without dirty hacks).
- Fixed: 726275 - Shift-click on back/forward button doesn't load page.
- Fixed: 712763 - Session restore loads saved windows in reverse order.
- Fixed: 608735 - DOM: XMLHttpRequest's getAllResponseHeaders() returns null if the request is the result of CORS.
- Fixed: 838269 - DOM: Support cross-global |... instanceof DOMInterface|.
- Fixed: 777385 - DOM: Support Paris bindings objects that are either nsISupports or non-cycle-collected as weak map keys.
- Fixed: 818023 - JS: function.caller does not always work.
- Fixed: 786135 - JS: Make parseInt("042") === 42.
- Fixed: 827784 - [Windows] Provide an option to disable favicons on webpage shortcuts.
- Fixed: 837859 - [Windows] Enable Windows Media Foundation support for H.264 AAC, and MP3 playback on Windows 7 and later.
- Fixed: 813488 - [Windows] Enable metro browser build.