This what I have installed
Adblock plus - blocks adds
British English Dictionary - dictionary for real English
Cooliris (formerly piclens) - that 3d slidy image gallery thing
Download Helper - helps download files from places like youtube.
DownThemAll - bulk downloader, works via keywords etc - very good
Fast Dial - a better version of the opera panels, more than just 9 links
FIrefox Showcase - think mac expose for firefox
Foxmarks Bookmark Synchronizer - syncs with an online server
Google Preview - quick thumbs in google
IETab - uses ie to render the page from within firefox
Linkification - converts non hyperlinked links to links
PDF Download - downloads pdf's instead of direct open
QuickPageZoom - little icons for page zooming
Tab Mix Plus - enhanced tabs
Tab Scope - little images of whats on each tab, bit like the vista pop up images on the taskbar
Tweak Network - tweaks network settings
febe - backup everything
imagezoom - allows you to zoom images independently of page etc (why firefox doesn't have something like ie7 scale/zoom images is beyond me)
Just added that split browser one, looks like it could be useful
