Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

What Firefox locale (user interface language) are you using in case this is a problem with that locale?

cor-el said

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

What Firefox locale (user interface language) are you using in case this is a problem with that locale?

I will try that, BUT if the only variable I changed was the version of FF I was running, why wouldn't that indicate a problem with FF itself?

As for locale, I think I'm in en-US, but I don't know how to verify that.

As for "Reset" button, do you mean the "Refresh" button?

I started it in Safe Mode, and Command-F worked.

Then, per https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectlocale=en-US&redirectslug=Troubleshooting+extensions+and+themes#w_the-problem-does-not-occur-in-safe-mode I turned off Hardware Acceleration and restarted normally. Command-F worked.

So I re-enabled Hardware Acceleration and restarted normally, and Command-F still worked.

So I have no idea what the problem was, but it seems to have gone away all by itself.