Interesting, it doesn't work for me either on mozilla.org. I think Firefox actively blocks content script injections on mozilla.org (though the documentation only says that's the case for only addons.mozilla.org).

Maybe try it on a different website, e.g. https://github.com/jlebon/textern/issues/new, and if it still doesn't work, feel free to create an upstream issue there :)

The default key is Ctrl+Shift+E, but note that you can change that mapping in the extension settiongs.