The plugins and stuff make Firefox for me completely different. Even when i go on a plain version of Firefox, say at colloge i go 'ekhh...can't use!'.
I'll just mention the RedShift V2 skin (no one mentioned not mentioning skins right?) because that's what also makes plain Firefox unusable to me lol.
Anyways:
All-in-one Sidebar - the layout went all weird in this at one point, might be my set-up or one of the updates.
Download Statusbar - puts all yer downloads in lil boxes at the bottom of the screen
Downloadhelper - helps in easily getting most videos off sites, works with some other flash like stuff aswell
Firefox Showcase - Shows all yer tas open in a preview window, never use or need really, don't even know the button for it lol
RSS Ticker - scrolling RSS feeds along the bottom, very useful
Smart Bookmarks Bar - Gets rid of all the text in the Bookmarks Toolbar, looks much better.
Must say thanks for making this thread lol, it's given me some great plugins to go off and get...now.