I've been using Opera a bit on and off over the years, but it's always been slightly unstable for me, and (especially back when I used it a lot, things are MUCH MUCH better now!) some sites didn't render all too well (I suspect that a big factor was the overweight of poorly designed IE-only pages back then).
These days, I can't really be arsed to switch from FireFox. Sure, Opera is
faster and lighter, which does matter a lot to me, but it still seems to crash every now and then, and there's no plugin support.
Took me quite a while to adopt FireFox, since I used Avast (better than Maxthon imho). But I finally got hitten by a drive-by exploit in a banner ad or something, and from then on I never looked back, although I reserve the right to bitch about FireFox bloat and (loading) slowness.
Never been a friend of "smooth scrolling", even in IE. Too slow, and scrolling in the opposite direction doesn't "cancel" scrolling, so you have to wait until the smooth scroll is done until you can scroll back - annoying. And it's absolutely crapalicious on terminal servers