Most extensions that exist are on Mozilla's Addons site. And unsigned extensions aren't commonly distributed because you just can't install them on normal Firef

Most extensions that exist are on Mozilla's Addons site. And unsigned extensions aren't commonly distributed because you just can't install them on normal Firefox, only Nightly or Developer Edition builds. The main use-case of this is so that Mozilla doesn't have total control over what extensions are allowed to be installed. It can also be used so you can install extensions you built-from-source.