the biggest problem is that browsers try to be everything these days and end up doing their main functionality in a firmly mediocre way
66% of the addons ive got installd are things that firefox should be able to do by default and if i had a 5 button mouse at work 100% of my addons would be of that kind

and then theres the problem with the crappy and annoying way the tab bar works in firefox

Quote from Dajmin :

My biggest problem with Opera is that a lot of sites don't support it. That's why I keep Firefox installed as well. If one doesn't do it, the other will. I was really pleased when I found out that you could add the mouse gestures plugin to FF too.

That's because these websites are coded sloppy. Opera renders strictly after the W3C-standards, so if websites don't comply with those standards, you will likely have problems viewing it with Opera...