This shouldn't be recommended by Mozilla. This is a non-libre add-on to store and sync online passwords.
And the owning company is based in the USA. They will (or have) received request for user passwords like Lavabit did for their encrypted email.
https://en.wikipedia.org/wiki/LogMeIn

Libre/Open source software should be essential to recommend an add-on. They are using Mozilla's work without reciprocating.

And it violates Principle 4 and Principle 7 of Mozilla's manifesto https://www.mozilla.org/en-US/about/manifesto/