No there are various issues and some are individual like this one. 7mins. Please uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.4 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Check

• https://www.youtube.com/html5

Update Video Card Drivers please

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

Do you have any YouTube, video or Flash related add-ons installed  ?

If so, would you disable them to see if that will make a difference  ?

Happy112's reply was the key. I'll post the particulars here for anyone else with a similar problem. The issue was competing add-ons. Details:

• Several extensions were added some time ago when Flash Player was becoming a problem and the transition to HTML5 was underway. Different extensions had different effectiveness for different videos, and I loaded several for evaluation, hence the multiple add-ons. The extensions coexisted peacefully and several were left enabled with no problem.

• The add-ons were all compatible with Firefox V57, so none were disabled as "legacy" extensions with the update to V57.

• The update to V57.0.4 appears to be the change that triggered the symptoms.

• The problem was due to two add-ons:

    - YouTube Flash Player by Maxime RF forces videos to use 
      the Flash Player

    - HTML5 Video Everywhere by lejenome forces use of the 
      Firefox native video player.

• When both add-ons are enabled under V57.0.4, they both work simultaneously, playing just the soundtrack in one format, and the video and soundtrack in the other format. The one playing just the soundtrack started immediately upon loading the video, often prior to any visual information being presented in the window. The one playing both started either several seconds later after visual rendering was complete or waited for the Play button.

• Neither add-on is now needed, so I removed both.

Youtube has not used the fallback Flash Player since end of July last year I believe so these Youtube Flash related extensions can cause issues since.

Pkshadow said

Please uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do. Reinstall with Current Release Firefox 57.0.4 with a Full Version Installer. Update Video Card Drivers please

Thanks for the reply. Going back to a clean setup is usually good advice. If it doesn't solve the problem, it gives you a clean starting point to work from. That was going to be my next step after checking add-ons. Luckily, the add-on problem was discovered first. If I had invested the time to clean up and start over, and had reinstalled the same add-ons, I would have ended up with the same symptoms.

But I appreciate the time you took to develop a comprehensive answer. It wasn't the solution in this case, but it's a good resource for future problems.

That reply was more of a clipboard posting and not a custom reply so much. A uninstall and reinstall of Firefox alone does little usually.