Back last August, as soon as Firefox 3.5 was released, I installed it on Ubuntu 9.04 (I can never remember the animal names). I didn't know how to do it, because I was a rank beginner. Still am. But back then I was about as rank a beginner as there is, because the first time I had ever so much as downloaded a program, even in Windows, was 4 months before that, in late April. In May, after reading a number of Ubuntu how-to books, I installed Ubuntu 8.10 and in July I replaced it with 9.04. In August I was dismayed to learn Ubuntu would not permit me to update Firefox automatically from 3.0.something to the brand-new 3.5. (I'm still not enthusiastic about this feature it reminds me too much of Microsoftian control techniques. More on this in a moment.)
I'm a reader of documentation, not so much a lurker on forums. Unfortunately, when I went looking for enlightenment I found nothing in ink-on-paper that offered me a prescription for performing such an upgrade myself. To this day I don't know exactly what steps I took to do it, but I recall it involved downloading directly from the Firefox site following their instructions, I think some degree of unzipping and moving it about, and in short order there stood a link to Shiretoko (Firefox-speak for 3.5) in the Internet section of my Applications menu.