To clarify… If I do a safe mode restart of my regular FF Portable install, with passwords and cookies for messenger removed… I then must do a manual entry of email & password, then as soon as it click login or hit enter, the entered password doubles and the save password prompt pops up.

I have found out that this password entry behavior may be messenger.com hashing my password as part of their End-to-End Encryption. This in combination with login issues led me to believe the password was being changed before being submitted. Since original post I've tried with a couple other browsers and get the same result. Hope this helps others who come across this behavior.