Firefox 4 beta 1 now officially out
lol that's the build I've been using for a week now
...okay they needed to test it...
https://www.mozilla.com/en-US/firefox/all-beta.htmlps: be warned, there's an annoying bug when using persona skins, if using the new, and default, title bar layout. I'll let you find out...you can solve it temporarily by switching to the old layout way, with menu bar displayed.
edit: you might want to read this page first
https://www.mozilla.com/en-US/firefox/beta/