This debate has been on these forums as least 4 seperate times. What it always boils down to is:
Opera is faster, Opera passes Acid2, Opera has mouse gestures, Firefox is open, Firefox IS BETTER !11!!, Firefox has many more addons availible, Opera has a lot of the most functional things out of the box, Firefox can be themed, by saying that you obviously havn't used Opera which can also be themed in any way you want, and so on and so forth.
That last point for FireFox is where people usually consider it to be way better. Firefox starts off as a relatively blank slate that lets you addon all sorts of whimsical things. Opera on the other hand comes to you with everything ready to go. Recently Opera has pushed for more third party bits and has MANY more than it had 1-2 years ago, but it's main point is still that everything is integrated.
I'm a long time Opera user (at least 6 years) and for me Opera is great because without having to hassle with anything, it has all the functionality I'd ever need from a web browser built in. I use Opera as not only my web browser, but my irc client, my newsgroup client, my email client, as an rss feed tracker, a bit torrent client (ok, so I don't but it can do it), and wether a page works right in Opera is almost always the litmus test to wether it was written correctly.
Some have made the compairison to me in the past that Firefox is like linux, you start off with something that is barely functional, but all the tools and bits are out there for you to make it exactly how you want. Then Opera is Mac OSX, you don't have to worry about how it works because it does everything you ever need it to do and does it with style, if you like it's style you'll love it. With that thrown out there I'll say I mostly agree except that Opera is way more customizable than OSX =P.
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