This is about the Firefox Add-ons I use. This really is a complete list, I like to keep my browser lean. I'm fully aware that most people will miss things like Adblock, Pixel perfect and greasemonkey, but I don't use them.
UsabilityTab Kit: My favorite tab extension. Has many many features, I use it mostly because it allow more than one row of tabs and tab grouping/collapsing.
Clear Fields: This solves a quite annoying issue with Firefox and the xorg clipboard. Xorgs lets you paste things that you have highlighted with the mouse by simply clicking the middle mouse button. However, the previous contents of the search and address fields don't get cleared. This adds buttons to clear those fields before pasting.
Gridfox: Indispensable while making grid-based layouts or aligning text to a base grid.
Web Developer: This is what I used before Firebug existed, and it's still useful today.
MeasureIt:This is very useful for quickly measuring the size of various elements on a page.
Privacy/SecurityCookie Monster: The best cookie manager I could find. I always block all 3rd party cookies, and by default no website is allowed to store cookies on my computer. Cookie monster makes it very easy to allow cookies for sites I visit. Another nice feature is to only allow session cookies for some sites.
Flashblock: Blocks all annoying flash animations. Can whitelist sites of course. A must.