The release candidate build of the Firefox 4 for Android and Maemo is now available to download.
Usability-wise, the release notes point out that there are now 150 add-ons that work with Firefox 4 for Mobile, up from 100 last month -- and there's now a Form Assistant, which make logging into websites a lot easier, especially if you've Firefox Sync enabled.
Finally, the RC contains a link to Firefox Spark, a new badge-collecting website, which seems to be one of Firefox 4 for Android's 'launch goodies.' Firefox Spark will be a social game, where you share the Firefox for Android download link, collect badges, and level up.
Here's a promotional video from Mozilla that demonstrates Firefox 4 for Mobile:
Download: Firefox 4 for Anndroid and Maemo