Thanks. I guess I could try uninstalling Firefox, but I have a hard time understanding how it would cause the computer to flicker even when Firefox is not running. On the other hand, I note that some of the people who had this problem have said that it started to happen after they'd been viewing YouTube videos in full screen mode. That's exactly what I'd been doing. For several days in a row, I'd watch an old movie on YouTube, full screen. Prior to that, I hardly ever used full screen mode. I actually thought that the video circuitry was failing, but after reading all of the reports, it seems to be a problem with the OS randomly changing video modes.
I would like to upgrade to High Sierra or later, but unfortunately I'm running at least one application that can't run on anything newer than Sierra.
I think maybe I'll try setting up a bootable external drive with High Sierra on it, and see if that fixes the problem. If it does, then I guess I'll go ahead and upgrade the internal drive, then set up an external drive with Sierra and my old apps.
