I’ve installed lots of Firefox extensions over the past few years but there are only 4 that I use regularly and can’t live without:

• Firebug
  The best Javascript and CSS debugger ever. I love the way I can just hover over something and see the attached styles.
• Del.icio.us Firefox Extension
  I use this everyday - multiple times!!! I use this extension to add all my links - http://del.icio.us/aneesha
• Screengrab!
  Just a time saver really. I can right click to capture a Web page - the whole scrolling page or just the displayed screen area.
• Selenium IDE
  Record functional tests right in Firefox for the Selenium functional test tool. Just select an element on a page and right click to view the available/applicable assertions. Also a great form filler! I’ve not used this recently but it was such a time saver and no brainer to use when I did have to do functional testing that it deserves to be mentioned.