I try to keep the number of installed extensions on my firefox as less as possible, since many of them have memory leaks which add to the already heavy RAM usage that FF is infamous for. Here is a list of my favorite extensions:
Del.icio.us – This is one thing I can’t live without. I use delicious for all my bookmarking needs and this is a great add-on, which adds two buttons on your firefox toolbar for easy access to your bookmarks.
DownloadThemAll – Great download tool which integrates well with the browser. Provides almost similar functionality to flashgot without the need for an additional download manager.
IE Tab – Many websites still strictly allow only IE browser – like most sites on my corporate intranet. In order to keep the number of open windows under control, you can use this extension. The latest version even allows you to automagically switch to the IE rendering engine for a set of pre-defined websites. This extension works only on Windows since it needs the IE browser.
Time Tracker – Very useful extension if you have the habit of browsing heavily at work. Check your internet usage at the end of the day, for a real surprise!
Google Notebook – Great plug-in to capture notes and store them online.
That’s it. Thought it is a short list, these little applets greatly improve the functionality of the browser.