DELL-Chris M - thanks for the tips.  But I have multiple P3421W's and have updated the firmware in all of them to M2B207, including to the subject monitor a few weeks ago.  So I'm familiar with the drill.  And I didn't try to update from DDM -- that's only how I learned that M2B208 was available. But I just successfully updated the firmware in the monitor. I tried using the firmware update utility again this evening, and it again failed to recognize my monitor.  I then intended to push the on-screen-menu button on the back of the monitor to do a factory reset, but I accidentally hit the power button instead.  Then I realized that cycling power on the monitor may be a good troubleshooting step. So, I tried the firmware update utility again, and it DID recognize my monitor, and the update to M2B208 was successful. I obviously wish I had thought of cycling the monitor power sooner. It had not been turned off for many weeks, or maybe even months. Go figure.
I bought a P3421W monitor about a year ago. I installed M2B207 firmware into it when I received it. I bought a second P3421W last month. I initially installed M2B207 firmware, then Dell Display Manager advised that a firmware update M2B208 was available. So I successfully updated that monitor to M2B208. All good for the new monitor. DDM later advised the firmware update M2B208 was available for my older P3421W. But when I tried to install it, the firmware upgrade utility says that M2B208 is not valid for my monitor. So, this looks like a bug with the firmware or the update utility?
P3421W, not accepting M2B208 firmware