Please give more details. Is this with just YouTube, or are there any problems on other sites? What does / does not happen? Does the web page load? Are there any error or other messages?

YouTube sends MP4 or WebM files to visitors with Firefox 33 and above.

• http://www.ghacks.net/2014/07/17/google-enforces-html5-youtube-firefox-33-newer/

Aren't all YouTube videos from YouTube???

The player just won't start. You can move the time pointer and it will show the appropriate stills. Reloading the page doesn't help.

There are no error messages. The HTML5 player is active.

Flash Player Test Page

http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-windows.html http://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-mac.html

You can try to disable OMTC and leave hardware acceleration in Firefox enabled.

• about:config page: layers.offmainthreadcomposition.enabled = false

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

Google has been using HTML5 with Firefox for a while so chasing after Flash doesn't make sense unless you also do the HTML5 avoidance maneuver.