There is a lastpass extension, including the binary version, for firefox:

LastPass Firefox Toolbar Version: 4.2.3 Built: Mon Dec 04 2017 13:51:36 GMT-0500 (EST)

Binary Component: true (Native Messaging version 4.1.44, built Nov 16 2017 23:33:27)

Is it broken upstream in the nightly build, but not in the stable build?