Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

If there is still a problem,

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

While you are in Troubleshoot(Safe) mode;

Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

How to disable Hardware Acceleration {web link}

Thanks, Fred.

This issue arose this morning immediately after I downloaded the Firefox update. I had no problem with black screens before, so I doubt it's corrupt cookies or cache. Something happened as a result of the update.

Did you try Troubleshoot(Safe) Mode?

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

You can change this pref on the about:config page from 3 to 2 to revert to the Firefox 94 behavior.

• layout.css.prefers-color-scheme.content-override
  Dark (0), light (1), system (2) or browser (3)

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

See also this pref:

• ui.systemUsesDarkTheme [0:light; 1:dark; 2:no-preference]

I'm somewhat of a novice when it comes to Firefox, so I'm not about to start playing with config files on my Mac. I'm annoyed that all I did was download an update recommended by Mozilla and that I have to deal with this stuff. I never had problems like this with other browsers. So I think the easiest thing to do is to delete Firefox and go back to Safari, which is much more reliable.

Thanks for trying to help.