On both of my Macs here Firefox started losing some of it's add-ons seemingly all on it's own. Apparently they're getting serious with add-on signing but as a result they're screwing up add-ons that are signed and designating them as legacy. Did some digging and found these two pages:
https://forums.informaction.com/view...100053#p100053
and:
https://news.ycombinator.com/item?id=19825830
Applied stuff from both and my FF add-ons seem to be back to working. Worried me that Privacy Badger and No Script both bit the dust because of this. But I have them back now.