I don't know, complete mystery. Fact is that the profile folder is in %appdata%\Local\Mozilla\Firefox\Profiles) (and then some in \Roaming\Mozilla\Firefox\Profiles), not in %userprofile%\Documents. I haven't seen any bug nor oddities as I keep it blo...
I've just installed the latest version, 137.0b2, and it also tries to access %userprofile%\Documents. I don't allow it via defender, and I haven't noticed any consequential problem, but the pop-up is quite annoying. Please Mozilla, what are you doing...