Firefox 20 was a pretty big release for the Web browser. It added a new download messenger and per-window private browsing, both of which are pretty substantial additions. Firefox 21 isn't quite as grandiose, but it does add a number of great new features for the Mozilla faithful.
Firefox 21 has two big features - a social API that supports more than Facebook and an improved Do Not Track UI.
The first is rather important as it opens up Firefox to more social networks. Lesser known social networks Cliqz, Mixi (Japan only) and msnNOW can now all be accessed right from a sidebar in Firefox without having to actually visit the site in question. It probably won't be long now until somebody builds a Twitter sidebar in Firefox with the social API.
The improved Do Not Track controls give users even more control over what they want advertisers to see. The previous Do Not Track dialog in Firefox only had two options - user says nothing and user says don't track. Those two options are kept in Firefox 21 with a third option explicitly telling advertisers to track their movements across the Web. It's a small addition, but an important one in the greater context of the Do Not Track debates.
Other additions in Firefox 21 include the Firefox Health Report and suggestions on how to improve startup time. The two kind of go hand-in-hand as Firefox Health Report will suggest ideas on how to improve performance in Firefox which may have a direct influence on application startup time.
As for Firefox for Android, here's what you will see in the latest version: