I would also recommend you do a search for these on the firefox addons site:
Adblock Plus - Remove Ads from sites
I've been using that one - Truly great!
plus
StatusBarEx - I feel much better about the MB/phisical number that it gives, over Task Manager's.
PDF Download- Another good one.
ColorfulTabs - Gives color for each pagetab, pretty :)
Fission - Progress bar in address bar.
Keyconfig - Rebind keys.
Gmail Space - Use gmail as FTP
IE Tab - To load in explorer engine instead, VERY usefull
NoScript - Pure genius.
Statusbar Clock - Nice to have clock when viewing fullscreen.
Sage - Rss Feed (not important)
Hyperwords - Lots more options, like the most usefull copy txt on select.
I look forward to trying these.
Those would be the most important ones, also remember you can type in about:config in the address bar to further tweak it. Let me know if you found any of these usefull :)
Wow! Hot tip; many settings to fool around with!