And now for something completely off-topic. Which firefox extensions do you use? I'm mostly interested in the extensions that help web development in one way or another.

• Firebug - Anyone who is into web development, and not using firebug should be hit the face with a stick or something pointy.</a>
• NoScript - If you are a web developer, you know you can't trust 90% of the other developers on the web. This extension prevents you from suffering from other peoples mistakes. As a bonus it also works like an ad-blocker. Because most ad-solutions use flash and/or javascript, you can easily block out certain domains.
• Web developer toolbar - Easy access to a lot of web developer tools. A lot of the features can also be achieved with Firebug, but I still like this one, because most tools are 2 clicks away.
• Whatmon - Monitor whatever. This is a really small extension that allows you to display a status of "whatever you want". All it takes is to generate a tiny XML document and you'll have it up and running.

Aesthetics

• Stop or reload - Merges the stop and reload button, because you never need both at the same time.
• Growl download notification - I like growl for all notification-style messages and I try to find plugins for every program it might be useful.
• History submenus - For a safari-style history menu.</a>
• OS/X favicon theme - This firefox theme is exactly like the default firefox theme, except that it displays favicons in your bookmark toolbars, like on Windows and Linux.