Firefox 4 Beta 12 has been released with new features, bug fixes and stability improvements. This release seems to be last beta followed by a release candidate.
I am still saying probably because of delays in release of Firefox 4 so far but as per the following comment from Mozilla release team and updated release schedule, beta 12 is the final planned beta for Firefox 4.
New Features in Firefox 4 beta 12
To add the PPA (daily build) and download latest beta, run following commands in terminal:
sudo apt-get update
sudo apt-get install firefox-4.0