截止到现在，Mozilla还没有公布Firefox 21的官方更新日志。不过，我们倒是可以从beta版一探究竟：

NEW: Enhanced three-state UI for Do Not Track (DNT).

NEW: Firefox will suggest how to improve your application startup time if needed.

NEW: Preliminary implementation of Firefox Health Report (see FAQ).

CHANGED: Ability to Restore removed thumbnails on New tab Page.

CHANGED: Add-ons History API removals in Places.

CHANGED: CSS -moz-user-select:none selection changed to improve compatibility with -webkit-user-select:none (bugs 816298).

CHANGED: Graphics related performance improvements (bugs 809821).

CHANGED: Removed E4X support from SpiderMonkey.

DEVELOPER: Implemented Remote Profiling.

DEVELOPER: Integrated, Add-on SDK loader and API libraries into Firefox.

HTML5: Added support for <main> element.

HTML5: Implemented scoped stylesheets.

FIXED: Some function keys may not work when pressed (833719).

FIXED: Browsing and Download history clearing needs unification to avoid confusion on clearing download history

另外，虽然Mozilla还没有推出Android正式版的Firefox 21，还是它仍然会在Google Play上为用户带来beta的更新。

下面是该版本的更新日志：

NEW: Shipping Open Sans and Charis fonts for Web Content.

NEW: Ability to save media files on long tap.

CHANGED: Polished UI based on Holo theme.

CHANGED: CSS -moz-user-select:none selection changed to improve compatibility with -webkit-user-select:none (bugs 816298).

CHANGED: Graphics related performance improvements (bug 809821).

CHANGED: Removed E4X support from SpiderMonkey.

DEVELOPER: Implemented Remote Profiling.

DEVELOPER: Integrated, Add-on SDK loader and API libraries into Firefox.

DEVELOPER: DOM/content implementation for <input type=’time’> Associated bugs.

HTML5: Added support for <main> element.

HTML5: Implemented scoped stylesheets.

FIXED: Download Manager page is not updated after clearing private data(777639).

FIXED: In content UI cut off on small screens(840593).