First i'd like to say its a great extension. And i know it is not targeted for FF57+ however it does install on FF57b2 however it doesnt work. First it doesnt check for new mails (it says "cant access dead object") and then in th eoptions i cant control watched labels since when i try to unselect inbox it says "observer is null" :)

P.S. on FF55 situation is the same. :(