Youtube's support: https://support.google.com/youtube/an.../3037019?hl=en suggests these steps.

More troubleshooting steps include:

Have you tried changing your DNS to 8.8.8.8, Google's public dns? It is also possible to disable Shockwave Flash player and use the HTML5 player in the meantime. (In about:addons change the plugin to "Always Ask") Please try to adjust the quality of the video as well. Clear the cache and cookies Right click on the player and select "Global Settings" and go to Storage and delete. Another way to try this is to open a private window and open the same youtube url. Did this help?

In order to investigate this issue further, please let me know if there are any steps that are confusing or need more help

If you have problems with current Shockwave Flash plugin versions then check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

Thank you both for the responses. Unfortunately, none of these made a difference. I tried all of those options from Youtube's support, but nothing changed. As for Fred's suggestions...I have already updated to the latest drivers and I already had Protected Mode and hardware acceleration turned off.

For what it's worth...I updated to 38.1.0 ESR and the problem has become less common. I just tested with a handful of videos and only half of them displayed the error (it had gotten to the point where it was happening with every video) and when it did happen, the message came and went so quickly that I almost didn't notice it. So...still not ideal, but at least it's an improvement.

EDIT: Nevermind, it's back to how it was. I guess it was just a fluke...

Does this happen in Firefox's Safe Mode? Just wondering whether any of your extensions might be blocking pre-roll adverts and causing this brief glitch, or doing some kind of rewrite on the player (some video recorders might not support the default player and may replace it).

By the way, you can confirm whether YouTube is serving the HTML5 player or the Flash player by right-clicking the player and checking the last item on the short context menu. If you get a long context menu (Firefox's default menu) in front of the short context menu, you can tap the Alt key to clear it so you can read the short menu.

It wasn't happening in Safe Mode last night, but I am using Safe Mode right now and it is still happening. It doesn't seem to happen when I switch to a new profile, however. According to the info page on Youtube, the HTML5 player is enabled. I have to leave for a doctor appointment right now, but when I get home, I will try testing a new profile with all of the same add-ons/extensions as my default profile and see if that still causes issues. Maybe it will help me narrow things down a bit.

After creating another profile and enabling all of my extensions, I wasn't having any issues, so I started going through all of the other files, one by one. When I got to the prefs.js, my problem was finally replicated. Just to be sure, I reverted that file back to the default and copied every other file from my profile over to the test profile. Everything was fine again. Then I copied my prefs.js file to the test profile and the Youtube error came back. So, it would seem that one of my preferences is the culprit here. The only problem is that I don't know if it's a preference tied to one of my extensions or to the browser itself. So...it looks like I might have to go through some of my "user set" preferences and play around with those. I'll start by searching for anything in about:config that mentions Youtube. If you might perhaps have a better suggestion, I'd love to hear it!

EDIT: No luck with that so far. Resetting all preferences mentioning "Youtube" didn't change anything. I did manage to find out that I had some remnants of an unwanted toolbar I removed a while back (Conduit), but removing everything related to that unfortunately had no impact.

I would be especially skeptical of customized preferences starting with network.http since those may make low level changes in how Firefox interacts with websites.

I know it has been over a month, but I thought I would report back to say that I have finally isolated the problem. To be honest, I have mostly been ignoring it because I've been too busy and tired, but I was able to spend a couple of hours going through my preferences and I managed to narrow it down to media.windows-media-foundation.enabled. Once I set this to false, the problem immediately went away.

Sadly, disabling it also means that I'm forced to use the HTML5 player, which normally wouldn't be an issue, but YouTube forces me to always view videos at the lowest quality level. I admittedly don't know why, but disabling media.mediasource.enabled seems to at least cause Youtube to play videos at 360p when available (instead of 144p). I'd prefer if it would play at 720p or at least 480p, but at least it's something. And if I ever need to, I can re-enable the "foundation" and it will force Youtube videos to play in Flash, albeit with the error message.

Hi ZombieToast, thank you for reporting back. When the MediaSource extensions are enabled, Media Foundation is enabled (and VP9 is disabled, which is the default configuration), usually YouTube will serve you MP4 in the HTML5 player. It sounds like that was mostly working except for the reloading.

As a short run workaround, either of these extensions will force Flash on YouTube:

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/
• https://addons.mozilla.org/firefox/addon/youtube-flash-player/

However, that doesn't seem to affect embedded videos.