Thanks so much for your prompt response. I think you've homed in on the problem. My 2019 Mac is currently undergoing repair at the Genius Bar, so in the meantime I've been using a mid-2011 Mac (10.13.6 OS - High Sierra) with Firefox 111.0.
After I posted my question, I realized that the OS on the older 2011 machine might not meet the minimum requirements. Apple TV+ needs a minimum of 10.15.* Watch on a Mac - Apple Support. Still, I was able to view the content, but I missed a lot of the dialogue without the closed captions.
Also (don't know if this makes a difference) I was using a generic Windows keyboard. I switched in my Apple keyboard, but it didn't make any difference in performance.
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* "Missed it by that much, Chief." [Maxwell Smart, Agent 86 https://www.youtube.com/watch?v=oPwrodxghrw]