SEE THE FIREFOX BLOG FOR DETAILS ON THE ERROR THAT'S PRODUCING THIS PROBLEM:
https://blog.mozilla.org/addons/2019/05/04/update-regarding-add-ons-in-firefox/

This review is 5-star because there isn't a problem with the extension. The current widespread problems have been caused by the actions of Firefox developers.

Firefox has already released a fix for this. It affected many add-ons, not just LastPass. Just go into "Firefox options", select "Privacy & Security", select "Allow Firefox to install and run studies". The update/fix is currently being broadcast this way. Close Firefox, Restart Firefox, give it a couple of minutes and your Add-Ons should reappear. Mine did.

Used this add-on for years, there was a brief period where it was broken on Firefox but it was an issue that Firefox had with a bunch of add-ons (and its been fixed now) so don't let the negative reviews fool you.