I still could recall Mick Genie ever guide you how to install Firefox 3.6.6 with Ubuntu earlier with post here.
Today, I am going to guide you on how to install the Firefox 4.0 Beta version.
1. Open terminal.
2. Run the command as below.
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-4.0
3. No step 3.