Firefox 4 is released recently. It also work on Ubuntu 10.10. You can upgrade it from PPA. Be keep in mind that some of the add-ons are not compatible with Firefox 4 at the moment.
HOWTO : Upgrade to Firefox 4 on Ubuntu 10.10
Archived thread 1718325 from Hong Kong Team. Markdown source: Hong_Kong_Team/thread_1718325_HOWTO__Upgrade_to_Firefox_4_on_Ubuntu_10.10.md