If you use extensions (Tools > Add-ons > Extension) like Adblock Plus or NoScript or Flash Block that can block content then make sure that such extensions aren't blocking content.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

Thanks. I tried restarting in safe mode and disabling Add-ons and the embedded videos still aren't showing up.

Try to disable the hardware acceleration in the Flash Player.

See:

• https://support.mozilla.com/kb/Cannot+view+full+screen+Flash+videos
• http://www.youtube.com/swf_test.html (right-click the player: Settings)

Flash "Display settings" window:

• http://www.macromedia.com/support/documentation/en/flashplayer/help/help01.html