Though this isn’t actually about importing extensions that aren’t already in Firefox, the extension APIs are standardised enough that this is actually possible a lot of the time. For example, I unpacked the Google Translate extension, made a single change to the manifest (I think so Firefox recognises it as supported) and loaded it into Firefox and it works perfectly! But it doesn’t work for everything, Amazon’s Send To Kindle extension didn’t work for me. It would be nice if there was an automated way to do this, I think there used to be. Maybe I can try to make something