Firefox is my browser of choice.
The most important reason is the add-ons. I'm a developer, and I have TONS of links and shortcuts, and the DEV toolbar. IE also has a DEV toolbar, but man, it sucks. ;)
With AdBlock, NoScript and Stylish installed, I also got used to no ads, anywhere.
I don't use Chrome, because the way it works just ticks me off. Tabs and menus are all in the wrong place.
Opera is OK, because it has a torrent download system built in.
And everybody knows IE is a dinosaur that needs to lay down in a tarpit and die.
have all of these browsers, plus Safari and OffByOne on my system, because I am a developer. I keep 'em up to date, give 'em a go every now and then, but for my everyday browsing, I keep coming back to Firefox. It's what I'm used to, despite it being a memory hog, despite Flash crashing all of the time (especially here on Yahoo)... I just like how it works and feels.