I'm definitely in the minority saying this, but every time someone convinced me to try firefox, I wasn't impressed. I run IE6 in full-screen mode all the time, so I don't have a need or want for tabbed browsing. Alt+Tab works fine for me.
When IE7 popped up as a system update for my computer, I just had a gut feeling and turned it down.
everyone has a good reason to switch to firefox... tabbed browsing doesn't work for everyone. try this.
get firefox 2.0 and install the following extensions:
i'm on IE6 right now, and it feels like i'm being pummeled by ads. once you browse for a few days without seeing any internet ads, you'll wonder how you put up with them for so long. filterset.G is an automatic updater that keeps the advertiser blacklist current.
ietab is great because in the not-so-rare event of a website that requires IE, you hit one button and the render engine automatically switches from mozilla to IE for that page. and you can set those pages to always render with IE.
i figure those three extensions have universal appeal, but once you get rolling with firefox, you can browse for other things that you might fancy. the power of firefox is in it's modability. i also use extensions for web development stuff and del.icio.us links. but if you can dream of something you wish your browser did, i almost guarantee you that someone has already made it happen with a firefox extension.