This does not happen with Safari, and it just suddenly started happening today with Firefox. My guess is that a setting suddenly got changed by accident. But there may have been an upgrade that does not work on my computer. 
 Any suggestions or ideas for further troubleshooting would be welcome.

When I navigate to a YouTube video, I get the word YouTube in giant font as shown in the attached screenshot. There is some gibberish along the left edge. Actually, when I navigate to a specific YouTube video, I watch the correct video show up normally for about 1/2 of a second and then it immediately converts to the attached screenshot. This does not happen with Safari, and it just suddenly started happening today with Firefox. My guess is that a setting suddenly got changed by accident. But there may have been an upgrade that does not work on my computer. Any suggestions or ideas for further troubleshooting would be welcome.