Ubiquity - This one lets you gets a command line interface to show up anywhere, anytime in your browser and lets you do several things like perform mathematical operations on the selected data, finding the place on the map, sending emails or tweeting updates by automatically using tiny url's among several other powerful things focused around user mashups and a simple way to lookup things on the web or visualize data the way you want to. You can even create your custom commands. Is the most awesome natural language processing tool I have seen and used for over a year and half on the web.
Del.icio.us - An awesome bookmarking service acquired by Yahoo! that lets you aggregate all your bookmarks and lets you sync them across different computers or mobile phones. So you can get to access all the sites you would like to at any point of time and never lose any information by just a single click.
Yahoo toolbar - It was there some years ago. Then moved out of use and now, is back with a bang! The ability to add custom apps and preview your emails on gmail or check your facebook pages by just a hover click while you are surfing any page on the web really helps avoid the frantic attempts to open new tabs and going to the websites.
YSlow - From a developers point of view really helps understand why a page loads slow and what could be done to improve your site performance in terms of front-end engineering.
Foxtab - Need that 3D experience in your browser? Install foxtab and get a glimpse of 3D tabbed browsing. Also to dress up your browser in a style you like use Personas for firefox.
Flashgot - Don't like the way you can't really get that video you watch on facebook or youtube or some file that's taking too long to load in the browser? Well, Flashgot lets you download any media to your system with just a click; Be it flash videos or anything else and that too without leaving the current page or stopping what you are currently doing. One of the best download managers for media ever!
Of course there are a whole lot of plugins that I haven't explored and another set which have been phased out of my browser. Yet the above plugins are what make my browsing sessions truly awesome!!
...and to end with a preview of what firefox is coming up with soon:
Comments and reviews of more firefox related addons are welcome!