I upgraded to the latest version of the LastPass plugin direct from the LastPass site and it seems to work fine. For whatever reason, the one they have listed on AMO is a whole major version behind.
Looks like you can either set browser.tabs.remote.autostart=true to soft-enable it, or browser.tabs.remote.force-enable=true to force it on. The wiki[1] does not encourage the latter, but suggests the former may only be available for beta (and more frequent) builds.