I use FF and have done for a good long while now. I tried Opera when they released a free trial or something (not too long ago). I didn't really like it. The theme was all 'XP-ey' and it seemed like it was trying to do far too many things (e.g. IRC client, email client etc.). Whats the point? I have my favourite software to do both of thsoe thank you very much.
I am a minimalist at heart, and i prefer to have the basic FF then customise it from there. Whereas with Opera it felt like i had to go round and turn everything off before i was happy. I'd consider myself an advance user (much like anyone here i guess), and it seemed like the week i'd delegated to give Opera a chance was spent more on fiddling about with the options, rather than actually using it. By then it was fairly similar to FF, so i thought there was little point!