These are the Firefox extensions I personally find them very usefully:
This is a Firefox extension that allows you to manage multiple gmail accounts and receive new mail notifications.
Gmail Skins is a Firefox extension that lets you change the look of your Gmail inbox.
Download videos from Youtube, Google, Metacafe, iFilm, Dailymotion… and other 60+ video sites ! And all embedded objects on a webpage (movies, mp3s, flash, quicktime, etc) ! Directly !
A simple CSS property viewer.
DownThemAll lets you download all the links or images contained in a webpage and much more: you can refine your downloads by fully customizable criteria to get only what you really want.
Autofill is a semi-faithful recreation of Google’s IE Autofill function in Mozilla Firefox (with some additional features).
ColorZilla provides a color reading from any point in your browser that can easily be pasted into another program.
DictionarySearch extension allows you to select a word, right-click on it, and choose Define. A definition provided by Dictionary.com then appears in another tab.