As someone else said, just install BOTH Firefox and Opera. Try them both out then decide.
That's what I did, and although Opera is a fine browser I went with Firefox. Linux + router + not being a dumbass installing anything that's offered by anyone (most important part) leaves me pretty damn secure so that's not an issue with either browser.
As for speed, Firefox + FasterFox extension = ZOOM ZOOM. (And no, I DON'T care that it increases server loads, cry me a river.)
Firefox has way too many great extensions for me to use anything else.
The most important thing is that whatever browser you choose, it's NOT IE. That's all that really matters. IEs browser market share going down = good times. I'm not some anti-microsoft person either *hugs xbox 360*