Adblock Plus - A No brainer

Adblock Plus Element Hiding Helper - Adds functionality to Adblock Plus

Download Helper - Invaluable

FastestFox - Probably one of the all around best add-ons out there

Febe - A must have for backups

Keyscrambler - Protection against key loggers

LastPass - A great add on that goes way beyond the normal password manager

Personal Menu - Add your most used menu items to one button and hide the menu bar forever

Read It Later - Rather than bookmark things you want to read with this addon, easy to add to the list and just as easy to remove when your finished reading.

WOT - A trustworthy add on that at least alerts you when approaching a bad site.