Tabbed browsing is good, and the bookmark thing Marc mentioned.
Better security partially because of the smaller profile and partially from being open source.
Better web standards compliance (IE7 is supposed to give everyone a run for their money here).
Themes are a nice bonus.
Extensions are awesome, it basically means there are an unlimited number of browser features. You can have a built in dictionary search, have the browser recognize gestures (do things based on specific mouse movements instead of mousing all the way up and clicking on a button), bookmark backups, the ease in blocking specific sites, images, etc (for ads and such), a cool right click menu, automatic passwords for registration required sites, etc etc..
As far as the bookmark thing, I've not experienced it but some searching says that more recent versions have seen a reduction in this problem.
There's nothing wrong with disliking firefox or saying anything against it (no software is perfect), just be prepared to say why (just as someone who hates IE should have to).
For free AV software I like Avast better than AVG:
http://www.avast.com/
And take anything you read on the grc site with a BIG grain of salt. His site is sensationalist and full of half truths and hyperbole. He's proven himself to be pretty much a self proclaimed security "expert" with no real knowledge.
Everyone should have a firewall (hardware or software like ZoneAlarm)