Mozilla Firefox 5 is the latest version of the Firefox web browser, released on June 21, 2011.This was the the first edition under the new faster-release regime Firefox kicked off in early 2011.
Even though the GUI has not been changed, Firefox 5 features:
Improved HTML 5 and CSS3 support
Improved overall performance
Visibility of Do-Not-Track option has been increased (disabling the ability for advertisers to track browsing habits and install cookies on the computer)
Stability and security improvements
How to install firefox 5 on ubuntu
Open the terminal and run the following commands
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get upgrade
Or if you are running ubuntu 11.04 you can try sudo apt-get upgrade it should install latest version of firefox.
Localize your firefox 5 using the following command
sudo apt-get install firefox-locale-$code
where code is the two-letter symbol of your language.