I found following extensions from firefox, which i found really useful.
Might be you are already using some extensions. If not, they are worth downloading and using...
measureIT -- measure pixel of objects on screen
screen grab -- grab screen (instead of using printscreen)
linkification -- convert text links into real URL
forecastfox -- weather information
web developer -- Loads of useful tools(Most important)
view rendered source chart -- important source chart
del.icio.us -- Bookmarks with Tags, very imp
flashGot -- download manager
pdf download
IE Tab -- open IE tab in firefox to test sites
Answers -- very useful to find out anything
sessionsaver 2 -- open firefox as you closed them
showIP -- show IP, domain etc, name servers etc
save image in folder -- save image in perticular folder
Extensions are small add-ons that add new functionality to Firefox. They can add anything from a toolbar button to a completely new feature. They allow the application to be customized to fit the personal needs of each user if they need additional features, while keeping Firefox small to download.