It plays for me. Here are links to some articles which may help. https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

Thanks for your suggestions. I have worked through the articles and have not yet found a fix. Follow up - one of the articles refers to RESET PREFERENCES. When I go to - Solution 2: Delete Firefox preference files

       Click the menu button, click Help and select More Troubleshooting Information. The Troubleshooting Information tab will open.  Under the Application Basics section next to Profile Folder, click Open Folder. Your profile folder will open. 

My folder has 4 different PREFS.js files numbered from blank to 3. Is this correct, or should I only have one pref.js file?

I understand that the numbered files are temporary replacements. They are mentioned in this article, which relates to a different problem. https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save#w_locked-preferences-file

I would be wary of 'resetting' preferences if that means renaming or deleting prefs.js. I would backup the profile folder before doing any such thing. Which article mentions that?

The first article - https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues Has a section that reads: Reset preferences See Reset Firefox preferences to troubleshoot and fix problems.

Following that link to a second article - https://support.mozilla.org/en-US/kb/reset-preferences-fix-problems

Contains the following directions: Solution 2: Delete Firefox preference files

       Click the menu button Fx57menu, click Help and select More Troubleshooting Information. The Troubleshooting Information tab will open.
       Under the Application Basics section next to Profile Folder, click Open Folder. Your profile folder will open. 
   Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.
   Click the Firefox menu Fx57Menu and select Exit.
   Locate and delete the file prefs.js (or rename it, for example, to prefs.jsOLD, to keep the old file as a backup). If you find more than one, a prefs.js.moztmp file or a user.js file, delete (or rename) these as well.
   You can close the profile folder and open Firefox now. 

Thanks for your continuing help. The link in your latest response goes to an article where it seems to say that it is OK to rename or delete prefs.js files in certain situations.

I had a disastrous experience of renaming prefs.js but, fortunately, had a backup of the profile folder. I would, however, try troubleshoot mode and follow up using the options in the article.

TNorth - I sincerely appreciate your ongoing efforts to assist. I did remove the extraneous prefs.js files from my system bu that did not resolve the issue. In the past day or two I have paid closer attention to what is, and is not happening. I CAN play embedded videos on news sites such as cnn.com and a Canadian site, CTV News Calgary. I CAN also stream various TV channels and streaming services, such as Canadian sports channel TSN and Global TV. However, what I CANNOT play is the videos embedded in the msn.com/en-ca website. The videos come from various sources that are identified as Kamera, Bang Showbiz, ET Canada, etc. When I try to watch these embedded videos on this website they will load, but will NOT play. I have checked all my Privacy & Security settings, including autoplay, and to me everything is set so that the videos should open and play. I have cleared (many times) the cache, and still no luck. In any event, unless anyone has another idea for a fix, I am going to give up and find another browser, I guess. One frustrated Firefox user. Thanks.

FWIW, I was able to play a video on that site. The one I tried to play was not available for my location but the next one (clicking the right arrow) played. I have blocked autoplay and have standard Enhanced Tracking Protection.

Well, not working for me. I have given up. However, as I previously wrote, I sincerely appreciate all your responses and attempts to help me. Thank you TNorth.