据悉，Firefox 21将在美国当地时间本周二正式对外开放下载。不过，现在用户可以在Mozilla的服务器上下载到桌面版的Firefox 21。如果是执念于官方版Firefox 21的用户，建议要随时留意在Firefox.com上推送的更新通知。
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).
