LastPass password manager is not listed in the add-ons/extensions. How do the add-ons/extensions get added?
Lastpass is for Firefox. There is no version for Thunderbird.
Are you looking for the the Firefox addon?
I have LastPass on all my web browsers(Google, Mozilla-Firefox, Microsoft Edge, IE and CCleaner Browser). I was looking to add it to Thunderbird.
Unless you regularly use all those browsers, I wouldn’t install LastPass - or any other password manager. In particular, I doubt it anybody is looking for or fixing bugs in IE: https://palant.info/2017/03/23/lastpass-security-done-wrong/