Firefox 4 Beta (version 1) 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.
What is new in firefox beta1 from here
Install firefox 4.0 beta in Ubuntu 10.10/10.04/9.10
Open the terminal and run the following commands
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-4.0