Firefox has just released the beta 9 version, not much new improvements are done. It claims to be faster than the previous versions. It is based on the Gecko 2.0 Web platform which improves its performance.
Read the complete changelog of the beta 9 from here
You can now install Firefox 4.0 in Ubuntu 11.04 / 10.10 / 10.04, open the terminal and type:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa sudo apt-get update sudo apt-get install firefox-4.0