tabbrowser prefernces (additional options for tabs)
firesomething (changes the name in the titlebar to something random like 'Mozilla Hypnochipmunk)
foxytunes (controls itunes, winamp, windows media player and about a dozen others)
showIP (shows the ip of the site)
colorzilla (gives you an eyedropper tool)
forecastfox (local weather)
ieview (gives you an option to open current page in IE)
and I use the brushed skin so it looks like safari.
oh and at the moment i'm also using
Portable Firefox which lets you run firefox off of a usb drive.