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
sudo apt-get upgrade