Hello Neophile, the new WebExtension mode of FF, which is the only one remaining (alice0775 was a XUL extension, this is not allowed anymore by FF), does not permit an add-on to function from within the bookmark library so far. So I didn't find any way yet to do what you suggest.
I tried also to see with alice0775 when publishing this follow up, but it appears alice0775 is not wanting to work with WebExtensions for now.