Firefox offers huge list of plugins/addons that let us to personalize our browser. I list some of them as very useful. Those are:
1) Firebug: Firebug is used to edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
2) MeasureIt: MeasureIt allows us to draw out a ruler to get the pixel width and height of any elements in a web page.
3) lori (Life-of-request info): lori tells us how long it takes to load a webpage.
4) IE View Lite: We use IE View Lite see how a site is rendered in IE.
5) Delicious Bookmarks: Its the official firefox add-on for Delicious.
6) Add to Search Bar: Add to Search Bar integrates any pages' search functionality available in the Search Bar of firefox browser.