This is for everyone who is impatient to wait till tomorrow i.e March 22 for the release of final version of Firefox 4.0. This Firefox 4 RC is considered to be stable and safe to use for daily web browsing, though the features and content may change before the final product release.
In order to test your favourite add-ons you have to install the Add-on Compatibility Reporter. The new version blacklisted a few invalid HTTPS certificates, updated localizations for 29 locales and added Vietnamese localization, bringing the total languages available in Firefox 4 to 83
To install Firefox 4.0 RC2 in Ubuntu 10.10 Maverick or 10.04 Lucid, open a Terminal and type :
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-4.0