A simple add-on that adds a link to the longdesc under images that provides one.

Maybe you want to look at this other extension for longdesc https://addons.mozilla.org/en-US/firefox/addon/longdesc/

The source code lives at https://github.com/Rik/longdesk

Disclaimer: I have no opinion on whether the longdesc attribute should or should not exist. I'm just tired of having to look through the source code every time I want to see a friend's explanation of a picture. This add-on has only been tested on http://www.nota-bene.org/Mise-en-abyme-a-carreaux

Also, this is my first add-on ever so…