Here is a guide to install Firefox 8 on Ubuntu 11.04. You can install Firefox 8 using the mozilla-daily ppa. This is still a testing version so nothing will happen to the current version of Firefox that you have installed on your system.
To install Firefox 8 open a terminal window (press Ctrl+Alt+T) and copy+paste the following lines:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
sudo apt-get update
sudo apt-get install firefox-trunk
To start Firefox 8 click on the Applications launcher and type: Nightly
Read more about Firefox 8.