Firefox users on latest version 41.0.2 noticed their browser as slow, lagging and freezes or hangs sometimes on YouTube when playing videos, it turns out that out Skype Click to Call added to Firefox 41.0.2 by recent Skype update has caused these issues.
Skype Click to Call add-on breaks YouTube in Firefox
If you’re using Skype, visit Firefox Add-ons page > Extensions and see if ‘Skype Click to Call’ add-on exists, if it was present, visit Control Panel and Uninstall Skype Click to Call to remove it from all browsers including Firefox.
If you don’t have C2C add-on in Firefox browser’s add-ons list or if the above solution hasn’t helped, then try to sign out of YouTube account and check does the issue persists.
If that also doesn’t help, then run Firefox in Safe mode by visiting troubleshooting information page, you can do that by going to about:support page and click on ‘Restart with Add-ons Disabled‘.