I used opera for a while, thats a great browser in my opinion. But recently went back to Firefox, and now I think its great!
I typically use Firefox and sometimes Chrome. Believe it or not, I haven't even upgraded my IE 6 to IE 8 on this older computer yet!
I use both IE7 as one of my browsers and Firefox 3.5.9 for another. IE7 was pre-installed in my computer when I bought it in September of last year. If I had a choice I would have turned it down. I had very bad experiences with IE8 in my last computer and rarely use my current IE7 browser.
Absolutely would never consider using any version of Internet Explorer. Its Firefox FTW most of the time. Sometimes Google Chrome. But you can't beat all the addons firefox brings with them.