Open local URLs ("file://") from any webpage. Please see the extension homepage for further information.
Note that MozDev has shut down long ago (mailing list etc). Add-On homepage, source code and issue tracker are on GitHub, as linked on this page.
Note on Firefox Quantum (57+): I have spent days and days trying to port this extension to the new "webextension API", but it I am afraid that new API is way too limited. I have not given up completely yet, although I don't see a good way forward.