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