Probably just a conflict w/ other extensions or something but, for me, LastPass's "beta" WebExtension quit working after upgrading from FF57 (developer edition) to FF58 (developer edition) on Arch Linux. It worked fine until then, though. It just throws an error saying my list of sites/passwords/whatever can't be downloaded and to try again. Likewise, the newer "non-beta" extension does the same thing.

Edit: Spent 30 seconds to troubleshoot. If I disable uBlock Origin and then login to LastPass (extension), it works fine. So just a small conflict somewhere. YMMV.