This post is over 3 years old, so please keep in mind that some of its content might not be relevant anymore.
The Firefox version bound with Ubuntu Lucid is still the 3.6.
It’s stable but definitely not the fastest browser I’ve ever tried and, these days, with Chrome/Chromium around we all know how important speed is.
So, the best way to keep your Firefox up to date with the versions released by the Mozilla team is adding their own repository with:
sudo add-apt-repository ppa:mozillateam/firefox-stable
Now you have to simply update your system with:
sudo apt-get update
and upgrade:
sudo apt-get upgrade
Enjoy!