Hold down the Shift key while pressing Del.

Helpful page on Thunderbird shortcuts. I find a new helpful one every time I read it. https://support.mozilla.org/en-US/kb/keyboard-shortcuts

OK, Holding down the shift key while pressing DEL DID work; however, in the past I thought I was able to right click the message, and click "Delete Message" while holding down the shift key. Since that seems no longer to work, I had wondered whether the shift key was not the correct key, and I tried different combinations of Control, shift, and Alt, again while right clicking the message and clicking "Delete message". Was the capability to permanently delete messages using (holding) shift and right-clicking the message and clicking delete been removed from Thunderbird?

Was the capability to permanently delete messages using (holding) shift and right-clicking the message and clicking delete been removed from Thunderbird?

I don't know if that did ever exist in the first place. Holding down the Shift key while pressing Del sounds a whole lot easier to me.

Since my right hand is on the mouse while the left can press a key, it is much easier for me, when I click to select an email, to simply then right click to delete. In most cases, this works fine, if all I want to do is send the message to the trash folder. But sometimes I accidentally duplicate one or more messages, and I don't the extra copies in trash, I just want them deleted. It's easy for me to select them with the mouse, and then right click and click delete while pressing a single key (shift) with my left hand rather than having to let go of the mouse and use two hands to press two keys, one of which is on the RIGHT side of the keyboard. What's easy for one person may not be so for another. And I do believe I was able to do it this way in the past.