The Copy Plain Text plugin for Firefox strips formatting from any text copied out of your browser window, and is useful if you move lots of content from the web into a word processor or spreadsheet program.
The Web Developer Toolbar for Firefox is a must for any web developer. It can outline elements (eg. div, p, table), display color information, show style dependencies, and much more.
SmoothWheel is a customizable addon to control wheel-scrolling in Firefox.