Firefox 4 has been officially released and is available to download @ http://www.mozilla.com/en-US/firefox/new/
If you want to run it on Linux, close any running Firefox instance, extract the downloaded archive and double click the file called "firefox" (not firefox-bin!).
Ubuntu: see how to install Firefox 4 from a PPA
Ubuntu: see how to install Firefox 4 from a PPA
To see what's new in Firefox 4, check out the following video:
For a complete list of new features, see http://www.mozilla.com/en-US/firefox/features/