I've tried all the recent versions put out by Mozilla and while they are better than I.E., I still prefer Opera.
The Mozilla browsers, to me, all feel "clunky". Opera seems to be smoother and better looking, besides being faster.
Just my opinion thought I'm a bit biased. I've used Opera for going on 3 years now.
I had the same feeling for about the first day. I downloaded some very neat extensions and have been in love every since.
Back when I used Opera, it was at version 6.50. It was alright, but it didn't feel solid. It was very configurable, but the browsing left something to be desired. 7.50 is much better, but I'm sticking with Firefox (which has a couple of tweaks you can implement to make it even faster than Opera.)
Mozilla itself is meant to be a web application suite, so a direct comparison to Opera doesn't quite work.
Any of the three programs (Opera, Firefox, Mozilla) are better choices than IE, but Firefox's customizability wins the day, IMO.
The feeling I get is that Opera's answer to features that don't work is to add new features that also don't work. It's a pity, because on paper it ought to be a world-beater.
Opera is my primary browser, too. I love the tabbed pages and suspect we'll see that feature in a new incarnation of IE in the not-too-distant future. I've tried Firefox but it loses out to Opera on little issues. For example, just selecting part of the address in Firefox's URL window is a pain compared to Opera. And in that area, nothing touches Opera's cool "paste and go" feature. Opera's only drawback is that it doesn't always handle pages well if they have non-standard elements like Flash or whatever.
MM