Firefox add-ons

Question

What Firefox add-ons do you use that are useful for programmers?

Answer 1

For web developing I use the Web Developer Toolbar, CSS Viewer and MeasureIt.

But I'm really not one of those who has a thousand of extensions to do everything. I like to keep things simple.

EDIT: Thanks to Dan's answer I don't need MeasureIt anymore. Can't believe I've never seen that! I guess I'll just have to pay more atention to this WebDeveloper toolbar.