If you like extreme bleeding edge for Firefox to the point of buggieness or even (temporarily) not working here is some info.
Gnomefreak has set up a feisty repository for testing the latest Firefox 3.0 builds, the repository is:
sudo apt-get update
sudo apt-get install firefox-trunk firefox-trunk-gnome-support
The Firefox 3.0 runs next to your original Firefox and can be found in your menu as “Firefox Web Browser (Development Version)”
Both Firefoxes cannot run at the same time.
For more information read Gnomefreak’s blog,