If Firefox has the following settings, you will get an "An unknown error occurred" error when you try to log in to Lastpass.
History Firefox will Never Remember history

If Firefox has the following settings, you can log in without any error.
History Firefox will Remember history

Firefox 81.0
Lastpass 4.56.1.3

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I can now log in after updating to Lastpass version 4.57.2.5 (September 28, 2020).

I have loved it and recommended it, and after a brief hiccup, I love it again.

Login Bug fixed in 82.0a

I've used this app and FF extension for a few years now and have not ever seen a single bug or had any complaint.. until now. Yes, they broke FF, yes, it is pretty bad. But one such bug in years of perfect experience? It's a 5 stars for me.
(But please fix FF, it's kinda pain ita right now to use it)