Whoosh, topic revival.
Opera 9.51 here. Personally, I think Firefox 3 renders pages a bit nicer, has a ton more applications, but the thing that ticks me off about it is that its RSS/Mail features aren't too hot. I've been stuck to Opera for a while now for a few core reasons.
1. The RSS Features are damn nice, looks like a compressed e-mail inbox.
Firefox has a dropdown menu for RSS feeds, and they're limited to a certain amount. Opera can take well... as much as I'd need. It doesn't cut off feeds I haven't read.
2. IMAP mail, works kind of like the GMail manager for Firefox, but it's built-in, and can be configured for any mail provider that allows for IMAP/POP connections. Works well, especially the IMAP. Deletes, reads, etc, directly towards the mail account.
3. Appearances/skins. Like any skin for any program, some of them are god-awful, but some are incredibly pleasing to look at. Opera has a good amount of eyecandy, but a bunch of the "top downloads" from Firefox give me a lot of disappointing looking interfaces.
That's pretty much it. From the websites I frequent, there aren't too many problems, but there comes a time and then some when I simply need to switch over to Firefox to get something to work (Javascript specific stuff sometimes, some other weird mishaps).
I can say though, Firefox is much more user friendly and widespread, so it makes sense that more people would vote for it. My vote still goes for Opera though, for the user interface and already-packaged stuff that I'd usually need a crapload of plugins for in Firefox.