The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
Session Manager saves and restores the state of all or some windows - either when you want it or automatically at startup and after crashes. It can also automatically save the state of open windows individually.
Notice: some important features (XML indentation and load old saved requests) are missing in version 2.0.0, please update to version 2.0.1.
Please follow the link ("View other versions") below To install version 2.0.1.
A Statusbar with speed, percentage, time and loaded size (similar to Opera's one)
Allows you to quickly switch to a different language (User Interface, Spell Checker Dictionary and Website content) in your Mozilla application.
Color tools for web development.
Color picker and eyedropper + saving colors and trying out colors with drag and drop.
Help make Firefox better. The Add-on Compatibility Reporter enables you to report if the Firefox add-ons that you're using are working properly.
This 15KB restartless add-on will add a "File" -> "Restart" menu item and a ctrl/cmd + alt + r hotkey to restart FIrefox or Seamonkey.
DOM Inspector is a tool that can be used to inspect and edit the live DOM of any web document or XUL application. The DOM can be navigated using a two-paned window displaying a variety of different views on the document and all nodes within.
A really generic user agent switcher, no restart required.