On what basis did *you* choose one over the other?
I still prefer the Mozilla suite. But that's old Windows days: I always used Netscape Communicator and then Mozilla under Windows. But I love the way FF handles rpms in Fedora.
For this list I use TB, but for my private mails I still use Mozilla. I used evolution with FC2, but now I just use TB/Mozilla. Old habits don't change easily.
I have Opera on this machine and have it licenced (I like those people from Norway). It integrates beutifully and uses all the Mozilla plugins (Java, Flash et al) without having to do anything. And Mozilla/TB are very restricted here, Opera not so, therfore I use Opera for sites where FF or Mozilla cannot handle JavaScript properly.
Also I found out that downloading the Mercedes Benz Mixtapes (fine, free music) throug a flash menue is awkward in FF/Mozilla and runs perfectly in Opera.
Otherwise: I just do not find a solution how to handle fonts properly with Opera - they are far too small by default, and I miss something like adblock. So it is not my default browser. But I will continue to support them (one of the few CS projects I really like).
-- Regards Markus Huber