If you check YouTube's diagnostic page, do you have red boxes?

https://www.youtube.com/html5

Not sure if anything in this article is helpful: Fix video and audio problems on Firefox for Windows N editions.

Do you use any programs that are supposed to record or download videos?

I do have a red box with an exclamation mark (!) in it on the MSE & WebM VP9 box. I don't typically watch youtube, and I don't know how to change that, or if that is the problem. I only went to youtube to see if I had the same non-playback problem with videos.

Nothing in that article was helpful for me. Not sure if because it did not address my problem, or because I did not understand some of what it was saying.

I don't know if I have any programs to record or download videos, but if I do, I do not use them.

spleeff39 said

I do have a red box with an exclamation mark (!) in it on the MSE & WebM VP9 box. I don't typically watch youtube, and I don't know how to change that, or if that is the problem.

VP9 is rarely used except by Google.

Since everything is detected as supported by Firefox, the results you're getting from actual players are puzzling.

Could Tracking Protection be a factor? When Firefox's Tracking Protection feature is blocking content in a page, a shield icon appears at the left end of the address bar. More info in this article: What happened to Tracking Protection?.

You can check the Web Console (Tools -> Web Developer) for messages about blocked content and about the presence of mixed or unsafe content.

• https://developer.mozilla.org/en/Tools/Web_Console

You can check the Network Monitor to see if content is blocked or otherwise fails to load.

• https://developer.mozilla.org/en/Tools/Network_Monitor

If necessary use "Ctrl+F5" or "Ctrl+Shift+R" (Mac: Command+Shift+R) to reload the page and bypass the cache to generate a fresh log.

Ok, when I go to the web console, I do get a couple of Red Alert type messages, but I have no idea what they mean or how to fix them. The first messages state: Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://pmp.ybp.yahoo.com/bid/yoo/adslot/144/?n=[timestamp&vi_ct=&vpaidt=&vpaidv=]. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Loading failed for the <script> with source “https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js”. r-sf.html:35 Loading failed for the <script> with source “https://js-agent.newrelic.com/nr-1071.min.js”. Then the ones in red, with what looks like a triangular alert type symbol says: XML Parsing Error: no root element found Location: https://t2.hulu.com/qos/playback?cb=1517365658208_906 Line Number 1, Column 1: playback:1:1 Ad errored out ERR_MEDIA_PLAYBACK Then the final note, in regular black font on white background states: Permission denied to access property "document" on cross-origin object ca:23:100 I am sorry, but while I know a how figure out some basic stuff, I am not a programmer or coder, or developer, and, while I can tell these are telling me there is some problems, I do not understand any of the information. Thank you for taking the time to try to help. I will check the Network Monitor now.

Did not see the earlier suggestion from Jscher2000. I turned off tracking protection, but it did not change anything. Still won't play. Funny thing is it worked fine the night before it didn't. Went to bed, and when I got up the next day and went online, it wouldn't work. Firefox did do an automatic update, but they did that on my laptop also, and it all works fine on the laptop.

'I found this solution in another feed. It seems to have solved my problem with the streaming video issue.

Chosen solution

Another thing to try : Try disabling graphics hardware acceleration in Firefox. Since this feature was added to Firefox it has gradually improved but there are still a few glitches.

You will need to restart Firefox for this to take effect so save all work first (e.g., mail you are composing, online documents you're editing, etc.,) and then perform these steps:

In Firefox 54 and below:

  Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
  Select the Advanced panel and the General tab.
  Uncheck Use hardware acceleration when available.
  Close Firefox completely and then restart Firefox to see if the problem persists. 

In Firefox 55 and above:

  Click the menu button New Fx Menu and select Options (Windows) or Preferences (Mac, Linux).
  Select the General panel.
  Under Performance, uncheck Use recommended performance settings. Additional settings will be displayed.

  Fx55Performance-disableHWA

  Uncheck Use hardware acceleration when available.
  Close Firefox completely and then restart Firefox to see if the problem persists. 

Did this fix your problems? Please report back to us!

If the problem is resolved, you should check for updates for your graphics driver by following the steps mentioned in these Knowledge base articles:

  Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems 

  Upgrade your graphics drivers to use hardware acceleration and Web