A few things to note:
Firefox uses the Gecko engine. It is supposed to render pages more accurately than any other browser engine, but it still can't pass the ACID2 test. the ACID2 test is a test set by the web standards project to test the accuracy of the rendering of pages by browsers. you can find it at
http://www.webstanda...g/action/acid2/ . The Gecko engine in my experience, is faster than IE's engine (called Titan i think, or Trident, or at least something beginning with T) and about as fast as the Presto engine (used by Opera). But its API allows for extensions, plugins etc like no other browser on Earth. It is also easier to release patches for. due to the way it's programmed. And because it's open source with a wide community, if you find a bug, it will be fixed extremely quickly, and you can expect to find support easily. You can also modify and distribute it, and claim it as your own. It also has many loyal fans, and many sites have been specifically designed to be best viewed in this browser. Some sites have been specifically designed to work best in Firefox (that's why some people may say it renders pages more accurately than Opera, although it probably doesn't.)
Opera, as mentioned above, uses the Presto engine (presto is italian for "fast"). It passes the ACID2 test, the second one after the KHTML engine (used by Safari and Konqueror). It's quite fast, but suffers from IE syndrome, where it waits until the majority of the web page has been loaded before displaying it. It's API does not allow for extensions as easily as Firefox does, and it's community is more narrow. And you don't get the freedom to copy, modify and distribute the browser like you do with Firefox, Konqueror and other GPL-licenced browsers. And you will have noticed it isn't as well supported either. And it doesn't have as many loyal fans as Firefox.
IE uses some engine starting with "T". It has an atrocious API, and several sites have to be specially designed for viewers to see them using IE. Because of its tight integration with Windoze, it takes a long time to release a patch, and they are usually of a much larger size than Firefox's or Opera's. Because you don't have the freedom to see its source code, you can't do anything about it either. It has a VERY small community and virtually no loyal supporters. Oh, and it fails the ACID2 test. (what a surprise).
My preference? Firefox. I just like it. Full stop.