I stick to firefox because of adblock. I like having the power to regex-match and strip out whole structural components of webpages. I haven't seen an ad in months, except in safari.

I would use opera (and acknowledge that it's better than firefox in a lot of ways), but there's about half a dozen UI quirks that I really really hate about it, with fixes ranging from a couple minutes (finding the right option in the prefs) to a couple hours (finding the js widget that changes the behavior to what I want it to do) to unfixable (the general feel of the ui, the tab ordering).

What's interesting, though, is that firefox is slowly trying to adopt some of those quirks that I hate -- like close buttons on the tabs instead of a single fixed close button by default.