I've been using this extension as a sort of replacement for the awesome SessionManager extension that stopped working after FF56 due to missing APIs needed (really, 'thanks' a million, firefox developers :-/). It sort of does it's job, and I don't want to sound negative towards the author, on the contrary, thanks for developing at least this plugin, under the various constraints, but it doesn't work as smooth as SessionManager did (once again, it can't, due to the missing features in recent versions of FF, hopefully this will get fixed over time).