Ubuntu 9.04 has FF 3.0.x and that's because they don't like to make any big moves so if you install updates right now you'll only get 3.0.13.
9.10 will have FF 3.5.x, it's out in October. You can also install 3.5 right now in parallel, package is called firefox-3.5. But then you'll have to run that one specifically cause 3.0 is still gonna be default I guess.