youtube videos can be streamed using HTML 5 plugin , openh264 video codec.

open about:addons and enable OpenH264 Video Codec.

Alternative:

install adobe flash player. https://get.adobe.com/flashplayer/

If you had already installed it , enable it in about:addons windows.

I have the latest Adobe Flash Player installed although it says it's 32 bit. I have Windows 8.1, 64 bit and had no problems until Firefox downloaded the latest edition, now I can't run any YouTube videos at all. Chrome and IE work fine. Firefox must have done something in their latest version to mess this up. So far unable to fix it.

1. In navigation bar type: about:addons
2. click on " check to see if your plugins are up to date"

Vulnerable plugins are blocked automatically by firefox for user safety.

Tried activating all plugins. Checked for updated Adobe Shock-wave, Flash player and reader all are up-to-date. Problem is still the same. All was fine until new version of Firefox, then unable to play any YouTube videos. Chrome and IE work fine. Problem is confined to new version of Firefox only. Windows 8.1, 64bit.

Adobe had released an update yesterday for flash player which addresses.

Version 17.0.0.134

Adobe wrote:

February 12, 2015

With today's release, we've updated the AIR SDK and Runtime with important functional fixes.

Download the new installer

https://get.adobe.com/flashplayer/

Alternative download link: http://www.adobe.com/in/products/flashplayer/distribution3.html

Followed your instructions to download latest Adobe Flash Player and whatever else they have for Windows 8.1, 64bit. The latest version of Firefox will still not play any YouTube videos. Just get the spinning doughnut then the " an error has occurred " message. Everything worked fine until Firefox released their new version so you guys screwed something up with your "new and improved version". Chrome and IE work fine so I guess it's no more Firefox until you fix this mess.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

   Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
   Do NOT click the Reset button on the Safe Mode start window 

   https://support.mozilla.org/kb/Safe+Mode
   https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Saurav, Thank you, problem solved. I have Yahoo Email, recently I started getting many obnoxious ads of all kinds on Yahoo. To stop them I have Ghostery and Adblock Plus, with Adblock came "Yahoo Mail Hide Ad Panel 2.43". In the fine print it states "Add-on does not work with Firefox for Android or any version other than desktop". I guess meaning 64 bit Windows laptops which I have. This lack of compatibility prevented YouTube videos from running. It does not effect Chrome or IE just Firefox, so for Yahoo Email users trying to block ads this is important. Thank you again, hopefully either Firefox or Adblock Plus will fix this. Robert

1) Enter the config by typing about:config in the adress bar. 2) Enter media.mediasource.webm.enabled and press enter. 3) Right click on it and change to true, for some reason it is set as "false" which makes MSE & WebM VP9 not work properly. 4) Visit: https://www.youtube.com/html5 and check that everything is OK. 5) Try to watch a video. It should work now.

Thank you for your help.

No problem.

If you would experience this problem again, there's another more drastic solution that work as well.

1) Go tho this page: https://support.mozilla.org/en-US/products/firefox 2) Click on "Refresh firefox". 3) IMPORTANT: You will loose the config-settings, addons etc which you will have to re-install/re-configure. But all the bookmarks, password, etc will be saved. 4) Now it will work.