Firefox 4 RC1 available to download, expected to be the final build
As far as the Firefox 4 development team are concerned, this is expected to be the final, release-ready build of Firefox 4; Mike Beltzner, Mozilla's Director of Firefox, says that this RC "represents the code we intend to ship as Firefox 4."
There are no significant changes between beta 12 and the RC. A lot of bugs have been squashed, and it looks like plug-ins are finally up to release-level stability.
Download Firefox 4 RC1 via FTP (HTTP mirrors will be available shortly)
Update: here's an HTTP download of the en-US build