InfoLister

About

InfoLister is an extension for Mozilla Firefox, Mozilla Thunderbird, and Nvu that collects various information about Firefox/Thunderbird and saves it to a file. Currently it prints the list of installed extensions, themes and plugins.

(from InfoLister homepage)

InfoLister is quite a good tool to save the list of your extension to a file that you may later copy/paste into or attach to your bug report.
Sometimes bugs are likely caused by some incompatible extensions, so it is good to have a list of extensions around.

Other times we developers might explicitly ask you for such a list.