Ever wished you could change the colour of Firefox, I havent but you can with this
Win only
https://addons.mozilla.org/en-US/firefox/addon/6991
If you're dead clever n stuff you might not need this handy status bar scientific calcumulator.
https://addons.mozilla.org/en-US/firefox/addon/6521
This is very similar to another extension called Cool Iris or Cool Previes, forgot which. It allows you to preview links without clicking an visiting.
https://addons.mozilla.org/en-US/firefox/addon/4999