User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:93.0) Gecko/20100101 Firefox/93.0

Steps to reproduce:

I have allways received two login promts in Thunderbird. This is annoying, but seemed to be related to having POP and IMAP accounts at the same time. After upgrading to Thunderbird 91.1.2 initially two login windows appear, but after entering the password in the first window, the second window moves behind the main program window and can no longer be selected. The only way to access this window again is to select "Minimize Window" in the Menu, enter the second password into the second password prompt window and then maximize the main window, again.

Sorry, this happens on a Mac with the current system (11.6)

You should find this is resolved with 91.3.0