Are you reading your mail in a dedicated window?

Yes. When I select a message in the message list and hit ENTER, a new window pops open. Pressing either the DEL or Backspace keys used to delete the message, but now nothing happens. Selecting "Delete Message" from the EDIT menu in this window DOES delete the message, but this is a lot of extra steps for a blind user who receives a lot of email. Once I close the window and return to the message list, either of these keys WILL delete the message.

Does anyone know how to solve this problem? I recently began having it, too. Current build is Version 128.1.0esr, 64-bit.

Sounds like something is stealing your keypress of the delete key.

https://bugzilla.mozilla.org/show_bug.cgi?id=1890567 reported something similar and restarting Thunderbird helped.