Firefox 4.0 (stable) has been released. Not officially, but is available for download via the Mozilla FTP servers.
The latest Firefox 4.0 stable is not yet available in any Ubuntu PPA and will most probably be uploaded to Ubuntu 11.04 Natty Narwhal official repositories firstly, tested and only then uploaded to the PPAs we were talking about last week.
If you want to run Firefox 4 in Linux already, you can download it from HERE, extract the downloaded archive and double click the file called "firefox" (not firefox-bin!).
I for one will wait for the PPAs to be updated because I've always encountered issues running Firefox downloaded via the Mozilla website, especially regarding the font rendering. However, these might be related to some packages installed on my computer so it might work great for you!
Download Firefox 4 | alternate download link (for Linux 32/64 bit, Windows and Mac)
Thanks to Carlos for the tip!