They're both great so it's hard to choose. I'll share what I think about each one:
Firefox:
PROS:
- Loads pages quickly
- More secure than IE
- Lots of themes and extensions to choose from
- Built in spell check
- Open Source
CONS:
- Uses a lot of memory with multiple tabs and extensions
- More flaws seem to be found as it gets more popular
- Could be a better selection of themes
Opera:
PROS:
- Lightweight for the most part
- Very secure
- Widgets are a nice touch
CONS:
- Not as popular as other browsers
- Interface is somewhat confusing if you've never used it before
- Isn't compatible with programs that use toolbars such as roboform
It's hard to choose which one is better. I think I'll probably have to say firefox since it works with roboform. I have both installed on my PC and I use both. Opera has built in torrent support which is nice so I use it for that sometimes, firefox has better add ons which I use on a regular basis. It is a close call though and it's hard to choose between those two.