Firefox 21 Final is available for download

Mozilla has released the Final version of Firefox 21. Firefox 21 extends the Social API to support multiple new providers. This version also enhances the Three-state UI for Do Not Track (DNT) and allows user to restore removed thumbnails on New Tab Page.

Release Highlights
Firefox 21 includes the following:

• The Social API now supports multiple providers: The Social API makes it easy for your favorite social providers to add a sidebar with your content to Firefox or notification buttons directly on the Firefox toolbar. The new social providers in Firefox 21 make it even easier for you to keep up with friends, family, news and events while you surf the Web. You can get real time updates about news activity, entertainment or your personal network while you browse the Web.
  
  New providers you can now add to your Firefox 21 include Cliqz, Mixi and msnNOW.
• Enhanced three-state UI for Do Not Track (DNT): Do Not Track includes three options "Do Track," "Do Not Track" and "no preference" to better represent user’s choice to opt-in or opt-out of online tracking. To access the Do Not Track options and indicate your preference click on "preferences" then "privacy."
• Restore thumbnails on new tab page: If you close a tab thumbnail on the new tab page there will be an option to "undo" and restore your thumbnail.

Download Firefox 21 Final:
The links in this section correspond to files available for this download. Download the files appropriate for you.

• Download Firefox 21 Final
• Download Firefox 21 Final for Windows (EN-US)

Firefox 21 (Running on Windows 8)

New features in Firefox 21
Firefox 21 has the following changes:

New:

• The Social API now supports multiple providers.
• Enhanced three-state UI for Do Not Track (DNT).
• Firefox will suggest how to improve your application startup time if needed.
• Preliminary implementation of Firefox Health Report (see FAQ).

Changed:

• Ability to Restore removed thumbnails on New tab Page
• CSS -moz-user-select:none selection changed to improve compatibility with -webkit-user-select:none (bugs 816298)
• Graphics related performance improvements (bugs 809821)
• Removed E4X support from SpiderMonkey

Developer:

• Implemented Remote Profiling
• Integrated, Add-on SDK loader and API libraries into Firefox

HTML5:

• Added support for
  element.
• Implemented scoped stylesheets.

Fixed:

• Some function keys may not work when pressed (833719)
• Browsing and Download history clearing needs unification to avoid confusion on clearing download history (847627)

Security fixed:

• MFSA 2013-48 Memory corruption found using Address Sanitizer. [Critical]
• MFSA 2013-47 Uninitialized functions in DOMSVGZoomEvent. [High]
• MFSA 2013-46 Use-after-free with video and onresize event. [Critical]
• MFSA 2013-45 Mozilla Updater fails to update some Windows Registry entries. [High]
• MFSA 2013-44 Local privilege escalation through Mozilla Maintenance Service. [High]
• MFSA 2013-43 File input control has access to full path. [Moderate]
• MFSA 2013-42 Privileged access for content level constructor. [High]
• MFSA 2013-41 Miscellaneous memory safety hazards (rv:21.0 / rv:17.0.6). [Critical]

Reference:
Firefox 21 Release Notes.