You have some Video Driver errors which can cause such issues re fro minfo sent : failures: [u'CP+[GFX1]: Potential driver version mismatch ignored due to missing DLLs igd10umd32 v= and igd10iumd32.dll v=0.0.0.0', u'CP+[GFX1]: Potential driver version mismatch ignored due to missing DLLs igd10umd32 v= and igd10iumd32.dll v=0.0.0.0']

Re-install or update your Video Drivers : https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration NOTE : Microsoft will update/change your drivers for Windows 10 Check your Device Manager, Video, Sound & Network Card https://www.howtogeek.com/263851/how-to-prevent-windows-from-automatically-updating-specific-drivers/

I do not see any add-ons/Extensions listed in the info that was sent. So that means you have none ? Try this and see if can do what you want on the sites https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode in safe mode. Or if Video drivers fixed the issue all the better.

You can also do this if feel it is a OS issue : Repair System Registry and Files

Run the Windows Deployment Image Servicing and Management (DISM) and System File Checker (SFC) tools as Administrator. The DISM and SFC tools scan the integrity of the Windows image and all protected system files replaces corrupted, damaged and incorrect versions.

Windows 10 To run the DISM command use the Windows key + X keyboard shortcut to open the Power User menu, then select Command Prompt (Admin). Type "DISM /Online /Cleanup-Image /RestoreHealth" (without quotes or copy/paste) and press Enter. While running DISM you may notice the process will appear to pause at 20%. After a few minutes the operation will continue.

(For Above/Below - Windows All Other Versions - Use Search To Find Command Prompt ** Enter CMD.exe to Find Then Right Click & Run As Admin )

To run the SFC command use the Windows key + X keyboard shortcut to open the Power User menu and select Command Prompt (Admin). Type "SFC /SCANNOW" (without quotes or copy/paste) and press Enter. For more information on using SFC, please refer to the Microsoft KB: Use the System File Checker tool to repair missing or corrupted system files

Save above for future use.

Please let us know if this solved your issue or if need further assistance.

This is a new hp laptop. No graphics driver updates available. Tried DISM; it said "no cleanup-image/restore health option available.

So then i tried the "scannow" option. That found some corrupt files and could only fix some of them. But it fixed the problem (to an extent: i must now double left-click on the screen, but that will make the labeling disappear,) Thank you for the help!

I would think you should figure out DISM on the HP site if it works at all for anyone. Very useful.

How about SFC /scannow ? What about the URL  : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode in safe mode.  ???? and running it in safe mode while you watch youtube vid that did not play properly. ???? Is that the only vid or is it all of them ??