The beta's been working fine for me (and it's actually 57+ compatible; it's just being held in beta until Mozilla releases 57). I wrote on my own blog how to get it to work if one has or had an older, non-beta version version of LastPass installed (the non-beta must be removed "completely" or it kind of messes up the beta; see here for more: https://marahmarie.dreamwidth.org/357152.html) but once that's done the beta seems fine (I've had literally zero issues in however many days/weeks since writing up that how-to).