download it directly.
Other than increased stability and performance when viewing Flash content, and some tweaks to hardware acceleration, beta 12 simply represents one last bug-stomping push towards the Release Candidate. The RC, incidentally, is due in the next few days.
With only a handful of bugs remaining, it is now almost time for Mozilla to pass its new browser over to the next standard bearers: the add-on developers. Most big-name add-ons now support Firefox 4, which is a relief -- but at the same time, very few add-ons are utilizing all of FF4's new UI elements. We struggled to find more than 20 add-ons designed specifically for Firefox 4, which is odd, considering there are "millions of beta testers." Still, with the final release of Firefox 4 around the corner, things can only improve.
Finally: it may please you to hear that, in beta 12, link tooltips are yet again displayed in the bottom left corner, rather than in the address bar! If you fancy some fun Saturday afternoon reading, there's an epic thread on Bugzilla that chronicles their return to the bottom of the window -- be warned, however: open source contributers sure are a feisty bunch...
[Thanks to Silver Wave for the tip!]