Firebug – Falls under top category and must have Firefox extension for web developers / designers, IMHO firebug is most efficient tool integrated with Firefox, a developer can easily edit, debug and monitor HTML, CSS and JavaScript at run time of any webpage

Grease Monkey – this extensions allows to write scripts, these scripts are handy and helps in fixing webpage bugs which makes your job simple and quicker.

Fire shot – This is my one of the favorite add-on, fire shot captures the webpage as screenshot, you may think that print screen is already available on keyboard, yes fire shot does the similar job as print screen and also it has most advance feature of capturing the complete webpage such as:

Print screen
Save screenshot as PDF/JPG/GIF/PNG
Fire shot editor allow to resize, crop, add text notes etc.
Undo/Redo
Upload screenshot via FTP

Page Speed – This extension is very compact, simple and most informative, tells you about the page load time.

Measure it – draws a ruler across on any web page, to verify the height / width