On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode.
On Mac: Hold the option key while starting Firefox.
On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
If Firefox is not running, you can start Firefox in Safe Mode as follows:
Click the menu button, click Help and select Restart with Add-ons Disabled.
If Firefox is open, you can restart in Firefox Safe Mode from the Help menu: Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). Try Firefox Safe Mode to see if the problem goes away. I have absolutely no problems with that site, but from the information I got from their page that says: 'Playback error: why is the player not working ?', this might be a good idea: