Firefox add-ons

Question

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

Answer 1

@Flávio Amieiro

MeasureIt is an unnecessary extension to have if you install the Web Developer Toolbar. Web Developer Toolbar includes a ruler as one of its features. Under the "Miscellaneous" category for Web Developer click the option "Display Ruler" to use a ruler identical to the MeasureIt one.

That will allow you reduce the number of extensions needed by at least one.