Thanks for posting your list of extensions. In past threads, this was linked to the PageTweak extension. You can try disabling (or removing) it on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then you can find it on the right side.

By the way, you have an unusually large number of separate ad blocking extensions. If any of those are unrecognized, consider disabling them because they might not all be what they purport to be.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that fix it?

Sorry, but it was of no help whatsoever. I was hoping something could be done in AdBlock or something to stop it. I DO NOT have keep-tube installed on my machine. However, something somewhere tells Firefox that it is okay to put that ad on every single embedded YouTube video I visit. What am I going to have to do? Backup my bookmarks (not sure how), uninstall Firefox, and reinstall?

This is SO aggravating and I can't believe Mozilla has done nothing about it.

Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Are the overlays gone? If so, the most likely culprit is another one of your extensions.