I should probably mention that the Opera browser is free now. I doubt Opera is motivated by alturism (I don't think they were making money off the desktop market anyway), it would be a worthy download since I still believe it is likely the best browser on the market.
Unmodified, FireFox isn't a big deal compared to Opera, but I have my FF customized big-time. If you are looking for an alternative to Internet Explorer (or if it broke), I would suggest Opera if you want a stock browser, and switch to FireFox if you want to add features or customization (or if you feel like bucking the system). Either way, I recommend avoiding Internet Explorer unless you absolutely must use it.