The wait is over ... Firefox 4 Release Candidate (RC) is available for download for Windows, Mac and Linux.
Here's the official announcement:
Mozilla Firefox 4 for Windows, Mac and Linux has exited the beta cycle and is now available as a release candidate in more than 70 languages. The millions of users testing Firefox 4 will be automatically updated to this version and will join our Mozilla QA team in validating the new features, enhanced performance and stability and HTML5 capabilities in Firefox 4. Testers are encouraged to check out the Web O’ Wonder in order to see the future of the Web with cutting edge demos that showcase the incredible online experiences developers can now create and users can experience. Developers can submit their own demos to the Mozilla Developer Network Demo Studio.
The important stuff from the release notes:
This Firefox 4 RC is considered to be stable and safe to use for daily web browsing, though the features and content may change before the final product release. At this time many Add-ons may not yet have been tested by their authors to ensure that they are compatible with this release. If you wish to help test Add-on compatibility, please install the Add-on Compatibility Reporter - your favorite Add-on author will appreciate it!
Download available here.
Release notes here.