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.

great app for firefox, that is until firefox has issues with the extension. Worked for years, today "kablewy" Bye bye firefox, if you can't keep up, I must move on