據悉，Firefox 21將在美國當地時間本周二正式對外開放下載。不過，現在用戶可以在Mozilla的服務器上下載到桌面版的Firefox 21。如果是執念于官方版Firefox 21的用戶，建議要隨時留意在Firefox.com上推送的更新通知。

截止到現在，Mozilla還沒有公布Firefox 21的官方更新日志。不過，我們倒是可以從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，雖然Mozilla還沒有推出Android正式版的Firefox 21，還是它仍然會在Google Play上為用戶帶來beta的更新。

下面是該版本的更新日志：

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).