If I have to choose between the two, I choose Firefox without hesitation. But I'll take Opera over either, without question, every time.
Yes i like Opera too, but i found that there were a few things that i was using that were incompatible with the browser so went back to the obviously disliked IE6!!
I keep Firefox handy for those few occasions where it's needed (I only use IE for Quickbooks online and testing, nothing else). But anymore, I'm finding it increasingly less common to come across sites that have been created so poorly they can't be used in a cross-browser environment. I love the convenience of Opera's built-in email client, so I do 99% of my browsing in Opera and save Firefox for those rare times when it's needed.
Mozilla Thunderbird - a next generation "IE" or another nail in the coffin for IE - lats face it - this poll's results must mean something!
out of the two .. firefox is definitely a better one bu even opera is also good as it easily blocks all popups .
although i used to be a feature-hog, i've noticed the degrading performance of firefox when using too many extensions, and its really hard to tell which extension is causing the most damage.