Because it needs to be approved, customised and otherwise vetted by the maintainers of Ubuntu Linux's repositories, the just-released Firefox 3.5 isn't officially available on that system. With one terminal command, though, open-sourcers can grab and install a working copy.
If you're hungry to give the latest Mozilla browser a go, here's the command you can paste into your standard terminal (with Ctrl+Shift+V, remember):
wget -O - http://releases.mozilla.org/pub/mozilla.org/firefox/releases/3.5/linux-i686/en-US/firefox-3.5.tar.bz2 | tar xj -C ~
As the Kabatology blog points out, however, you'll want to back up your existing Firefox profile, disable compatibility checks for rare extensions you don't want to lose access to, and create a new launching point for Firefox 3.5, as Firefox 3.0.11 will remain on your system until Ubuntu's official repositories update. All that is explained at the link below.
If you are trying out 3.5 on your Ubuntu system, tell us how it runs or what bugs you've come across in the comments.
A Single Command Install of Firefox 3.5 on Ubuntu [Kabatology]