Black Screen of Death?

Brand new iMac 27' running the latest and greatest Big Sur. I went into Mail to add the New Message icon (which somehow disappeared in the process of loading the new system from a Time Machine backup form my old iMac.) All of a sudden, the screen went blank. I had to do a hardware reboot. I went into Mail again to try the procedure, same result. Hardware reboot. Tried again, and this time it finally worked.

Then I was working with Firefox, and got another blank screen, which also required a hard reboot. It's happened two or three times with different apps; no consistency. Any clues?