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).