The Firefox version bundled with Ubuntu Lucid is still 3.6. It is stable but definitely not the fastest browser around, especially with Chrome and Chromium raising the bar on speed.
The best way to keep Firefox up to date is to let Mozilla do the work with their own repository.
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get install firefox
This adds the Mozilla team PPA and installs the latest stable Firefox. After that, updates come through the regular apt-get upgrade cycle.
Enjoy!