Firefox add-ons

Question

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

Answer 1

Groundspeed, is useful for testing server side code. It was created for input validation tests during pentest, but can be useful for any test that require manipulating input (similar to TamperData).

It lets you control the form elements in the page, you can change their type and other attributes (size, lenght, javascript event handlers, etc). So for example you can change a hidden field or a select to a textbox and then enter any value to test the server response and stuff like that.