First lets see if it is actually Firefox. Go to https://www.youtube.com/html5 and click 'Request HTML5 player'. Then test it.

Still not working.

The HTML5 player is currently used when possible.

This started right about the time Firefox was upgraded to 36.0

Separate Issue; Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 10.1.13 Adobe PDF Plug-In For Firefox and Netscape 11.0.10

Having more than one version of a program may cause issues.

Security Issue: Update your Flash Player v16.0.0.296
https://www.adobe.com/products/flashplayer/distribution3.html

I already have that version of flash installed and I disabled the plugins. Still not working.

I ended up uninstalling and reinstalling Firefox and it is working now. I should have tried that before posting. Sorry for the trouble.

Thank you for the suggestions.

When I reinstalled, I installed version 35. After I posted, I installed version 36 and YouTube will not work. It turns out that 36 is the problem I went back to 35 and it works fine.

I use YouTube extensively for research, so this is is important to me, and I would rather use Firefox than IE. For now, I can do without the beta.

zienkyz said

When I reinstalled, I installed version 35. After I posted, I installed version 36 and YouTube will not work. It turns out that 36 is the problem I went back to 35 and it works fine. I use YouTube extensively for research, so this is is important to me, and I would rather use Firefox than IE. For now, I can do without the beta.

The Beta version many times has problems. I suggest everyone tries to use the normal version before posting here. (Hense Beta. Its the pre-release that has many bugs)

This solution worked for me:

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.