Firefox add-ons

Here's a list of cool Firefox add-ons which are really useful:
Xmarks - The best add-on I've used till date. Synchronizes bookmarks of your browser and the account created in Xmarks. So when you use a browser elsewhere you can still get to your bookmarks and update.
DataFox - Gives the current usage of BSNL broadband.
TwitterFox - Easy way of accessing Twitter.
Adblock Plus - Blocks all unwanted ads in a web page.
Read it Later - Allows you to save web pages for later reading.
Persona - Cool skins for your firefox browser. Using Groovy blue theme now. Looks nice :)

Some others do exist which I don't use much. More add-ons as I add them...