Personally, I use firefox, but for a many good reason.

1) I used to use opera, but I eventually went on a guilt trip about stealing software (my key was stolen). I am a poor college kid and couldn't afford to pay for my web broswer so I had IE for a bit (firefox wasn't out yet). The best thing about opera is how you can left to right click or right to left click to go forward and back- Either way... opera is great but it costs money.

2) Since that time of my life I have come to love opensource and everything it stands for. I love how anybody can look at / modify / find flaws in code. It is basically an emergent system that creates the best possible software humanity could create... that is if everyone would just get into it. Firefox is free and opensource.

3) I am unsure of Opera's status, but as a webdesigner I cannot stand stuff that does not cohere to standards.. Firefox coheres to standards.

4) The main advantage of opera.. that being that it is fast... has become negligible cause my computer could launch a 400mb gorilla just as fast as two web browsers who are both light on system resources.

5) If you use multiple-platforms. (i use linux, windows, and osx on regular basises) Firefox is available on all of em... So you have a good amount of consistency and thus maintain effecient web browsing. Not to mention all the config files are compatible. I checked to see if its compatible (opera) with osx, and it looks kinda shadey... MacOS... I assume that means pre unix based macos... I never have run that so it doesn't help me.

The fact is they are both awesome web browsers and both deserve a harty hand of applause.. It is just circumstance which dictates which ones you use.