User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:82.0) Gecko/20100101 Firefox/82.0

Steps to reproduce:

With no unread messages remaining in the currently selected folder, type "n", which brings up a confirmation dialog saying "Advance to next unread message in ...", with a "Yes" button and "No" button with the "Y" and "N" characters underlined indicating they should work as keyboard accelerators for the buttons. Typing "Y" works and TB advances to the next unread messaging the the folder, but typing "N" has no effect.

Actual results:

Typing "N" has no effect.

Expected results:

Typing "N" should have the same effect as clicking the "No" button (i.e. closing the dlalog and leaving selected folder and message unchanged).

Using "N" as an accelerator worked until a recent upgrade. I'm seeing the new broken behavior on multiple Windows 10 machines with both 32 and 64 bit Thunderbird (78.4.3) and debian/linux 64 bit (78.4.2), but it might have started in an earlier release since I noticed this approximately a week ago).

Rob, does this also happen for you?

I can confirm this.
And I think it might be bug 1625632

Yep