Some nice add-ons:
Window Resizer - resizes Firefox so you can test to see what a web page looks like in all of the standard resolution sizes.
Supports the 640x480, 800x600, 1024x768, 1280x800, 1280x1024, 1600x1200 resolutions.
Font Finder - Get all CSS styles of selected text in Firefox.
FlagFox - Displays a country flag for the current website, server.