I've always been an Opera fan.
Don't get me wrong, Firefox is good, I just prefer Opera because it has built-in features I use which are only implemented on Firefox with extensions that can be buggy, slow Firefox down and don't always work with newer versions.
I think Opera has better theming than Firefox but I don't us it. I just have it set to use the same theme as Windows because I prefer all the programs I use to look similar.
For me, the only advantage Firefox had was Adblock but then Opera implemented it too, albeit not so well (no subscription to a site which keeps a list of annoying adverts which is updated regularly but a list which you need to keep up to date yourself) but that doesn't bother me as long as I can block the annoying adverts on a certain site I use. I have no problem with advertising in general and think it's good for webmasters to recoup their server charges without having to make people pay to use their site but content links (annoying pop ups which appear when the mouse is moved over certain pieces of text), large animated flash objects and irritating sound effects just push me to use Adblock.