I've been using Firefox for a couple of months now(after taking VoG™'s advice)and it would take a lot for me to use another browser now.
With all the extensions you can add and customize it to how you want it(instead of a large download with items included that you'd never use!)is one of its strengths. You have a browser that does what YOU want it to do.
An added bonus, that's been in the limelight recently, is with regards to security issues. Most exploits are targeted at internet explorer and so Firefox is safer to use. Admittedly, there was a security flaw found with it a couple of weeks ago, but unlike microsofts responses to security flaws, a patch was available quickly(hence the release of version 0.9.2.).
As to using Opera, I have tried it and it was quick, but I found the look of it cluttered and it seems to take up too much of my monitors real estate.
Only one minor niggle, I wish it could stay in memory all the while(as per Mozilla)so it would load quicker. (Got round this by having Firefox load a boot up and set this process to run at High every time I open it).
Sorry for the "War and Peace" posting but I think this is a GREAT browser, daz.