Ubuntu loads 3.5 firefox for you during installation, it also finds your hardware, installs the drivers and things just work. I have some complaints about Linux in particular but mostly they are personal complaints. Ubuntu works well and is the best distribution of linux I have used. I have to admit though that the last time I used linux was back in the Red Hat days and things have improved considerably. I still use XP for my main computer(another one has(ugh)Vista on it, it works fairly well, but somewhat slower. Unbutu is fast but the Movie player needs plugins installed, which takes up disk space, in order to run most DVDs, and you can't use Netflix instant play with it.
I haven't used Windows 7 and don't plan on it as XP still does what I want and I see no reason to spend money right now just to upgrade and make MS a few more bucks.
Try Mint. It installs all the codecs and Flash for you from the start. Otherwise, it’s basically the same as Ubuntu save for a couple of different menu features.
The last version I installed (Jaunty) only installed the 2.0 version of Firefox by default. That may be different now, but if that is the case, all you need to do is install the 3.5 along side.