The three-state UI for Do Not Track feature is now available, allowing users to tell sites if they want to be tracked or not, or simply express neither of the two choices (“do not tell sites anything about my tracking preferences”).
Also new in the stable release is preliminary support for Firefox Health Report, a feature designed to log health details about the browser. This will be used for further improvements in Firefox. It is turned on by default but users can disable the feature at any time.
In Firefox 21 Mozilla introduces suggestions for improving overall startup times and performance of the browser.