Great add-on, BUT;
problems with dec 6th 2018 update.
Im Still getting alerts as normal- but when i click the little toolbar "envelope", just will not load.
White "window" appears with blue "loading/Reload" circle - but doesnt load. Waited 5 mins... and still window is white with blue loading circle just spinning and spinning.
Had to roll back to v21.2.3.....
..... Which is working fine.

Win 7 [64bit] - Firefox v56.0.2 [The Last version before Quantum]