As long as we're both clear on one thing: both Opera and Firefox beat IE.
Then, it's a matter of personal preference. I really don't think one browser is truly overall better than the other. For me, Opera is definitely the way to go. Opera runs faster, it displays pages faster -- and almost never displays them incorrectly, and Opera has all the features I need in a browser. And it has them integrated much more smoothly than Firefox, for which you have to download extensions that not only slow things down but don't work as smoothly together and could cause conflicts. I also favor Opera's interface over Firefox's, mostly because Firefox's reminds me way too much of IE. Opera is also much more customizable than Firefox -- so even if you hate Opera's interface, you can easily change it to look completely different, and the skinning process with Opera couldn't be easier. Hundreds of skins are easily available on the Opera community website, and Opera automatically equips them once downloaded. So you really can't even complain about Opera's look or interface, because you can make it look however you like.