One day... Firefox 61 will fix a bug, and that will make implementing this easier. Hopefully some time after that.

*update: I tried to make it but alas, one cannot open the popup from a "doubleclick" event. That requires a User action, as defined here: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/User_actions . So the double click trigger is on-hold for now.