My goodness. I hadn't even thought about opera.
But, hang on... is there any reason to?
It used to be the case that it was the fastest, and I think for a while it really was. I moved from IE->FF->Opera.
However Chrome (and Iron) came out and kick its ass for speed now. I think Safari is now quicker too.
Opera doesn't have the community that FF has, and as such there are fewer of everything e.g. skins etc.
The other issue is that more often than not web designers don't bother testing with Opera, and it often shows. Lots of pages sometimes don't load properly / with full functionality, or even occasionally refuse to load as it doesn't recognise or "accept" the browser. There is a handy "Open with xx.." in the right click menu on Opera though; Opera kind of acknowledges there is a problem.
Another slight annoyance is the lack of options during setup. A lot of people here including myself would like more questions during installation, so as to remove any bloat or features that are unused e.g. the bit torrent part.
There are a few handy things in it (not sure if these are available with FF, probably is through add ons) like the notes section.
Opera overall isn't a bad browser, it's still good enough to warrant me to still use it. It was (and probably is) still one of the best for new ideas. Things like sessions and tabs started off in Opera if memory serves correct.