sudo apt-get install libstdc++5 libnotify-bin
Download ubuntuzilla .deb package or Visit the download page to confirm if it is the latest version.
Simply double click on the just downloaded deb package to install ubuntuzilla.
Run ubuntuzilla.py in command line by simply typing the same in Terminal.
When you are prompted to choose the language of choice, if you dont know the one you want, just type 14 there(which is the default en - US).
Thats it. Goto Applications > Internet > Firefox , and you will have your latest frefox 3.5 up and running, and will even prompt you with latest updates(you can disable this feature anytime