Adblock Plus :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/1865
Synopsis: Blocks out a cacophony of internet advertisements. Also allows you to manually 'Adblock' images via right click.

Video DownloadHelper :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/3006
Synopsis: Save videos in FLV format!

WOT :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/3456
Synopsis: An icon shows you what website to trust and what not to enter.

ColorfulTabs :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/1368
Synopsis: Fashion forward application that colors your tabs!

DownThemAll! :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/201
Synopsis: Get faster, multiple downloads at the same time!

FoxyTunes :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/219
Synopsis: Listen to music without opening up your regular media program.

ReloadEvery :: Firefox Add-ons
https://addons.mozilla.org/en-US/firefox/addon/115
Synopsis: Get more pageviews on your myspace!

A couple of good ones there. Out of that list, I use:
AdBlock Plus,
DownThemAll, and
ReloadEvery.

And I use a Real Player addon which is essentially the same as the Video DownloadHelper...

Here's two of mine I'll share:

Download Statusbar - https://addons.mozilla.org/en-US/firefox/addon/26
View and manage downloads from a tidy statusbar - without the download window getting in the way of your web browsing.

Extended Statusbar - https://addons.mozilla.org/en-US/firefox/addon/1433
A Statusbar with Speed, Percentage, Time and loaded size (like in Opera)


There's a whole lot more I use on my firefox that I've got on the computer at school, but I don't have as many on my home computer.