Works for me on TB v143.0.1 with either TB menu bar > Edit > Undo or ctrl-z.

Does the same happen in Troubleshoot Mode?

Edit based on Robs HowTos's comment:

If you don't see the menu bar on top, press the ALT key.

I am a longtime Thunderbird user. In the latest version, however, I cannot seem to find an "undo" command to correct a mistaken action while sorting new messages, such as a finger-slip that saves a message to the wrong mailbox or deletes a message I meant to save. Is there an "undo" for the last action taken?

I had the same problem. If you right click on the top line, you just need to check "menu bar". For whatever reason they set it as default off. "Undo" is still there. The answer you received caused me to finally search it out myself.