Personally I've used the following browsers:
IE 6
NS 4
NS 6
Mozilla Suite 1.7.3
Firefox 0.8-0.10 (Preview)
Opera 7.54, 7.60 (Preview)
Maxthon
Advant Browser
and a slew of other browsers which are based of the IE core.
I still prefer Firefox among all this browsers because, simply, it is standard compliant and yet it doesn't break as many pages as Opera does. I like Opera interface but it doesn't render too many websites as well as Firefox and it has problem with Gmail (which I uses). Preview 7.60 allow us to login to Gmail but it still doesn't fix the problem of rendering.
Granted that Firefox also got its quirks when it comes to rendering website, but most of the time it is because of bad HTML coding. But Firefox display enough of the websites around fine enough for me and as such, it is still my primary browser.
As for IE, security is really one big issue which totally help push me away from it. You may say that Firefox also got its security issue, but IE still sticks out like a sore thumb to me. One of my friend learnt his lesson the hard way when he got a trojan that was infected via a website (can't really remember the name, but it was the one that allow the trojan to be installed to your computer via a visit to a website that was hacked into, without any user intervention required). He now uses Firefox too.
Firefox isn't really a hard browser to get use too also. I've managed to convert a lot of my friends to use Firefox and none of them has regretted the conversion. In fact, most of them seems to like Firefox a lot more with its extensions.
Opera is a good browser to watch for, but until it can solve most of the rendering quirks, I will still stick to Firefox.
Just my 2 cents from Firefox.