Since some older hardware (Toshiba Satellite A300 for example) has issues with the current version of Grub as well as the newer Kernel I still need to run 10.04 on some machines.
However since the 10.04 Repository still uses Firefox 3.6 you need to add a PPA repo to upgrade to Firefox 4.
Either go to Ubuntu Software Center > Software Sources and click the ‘Other Software’ tab. Press ‘Add’ and enter ppa:mozillateam/firefox-stable
After adding the PPA you will be prompted to update your sources. Once done you can head to System > Administration > Update Manager to perform an upgrade
Alternatively you can do this via Terminal (Applications > Terminal).
add-apt-repository ppa:mozillateam/firefox-stable apt-get update apt-get upgrade