Could you check your plugins and make sure they are all up-to-date? Try installing the Adobe Flash Player from the Adobe website if you can't upgrade them via Plugin Check and let me know how it works out.

Didn't change anything.....

You can try to disable or uninstall the VLC plugin as that plugin can cause problems.

• https://support.mozilla.org/kb/Troubleshooting+plugins

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration 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.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

After trying everything listed, I still am having the same issue. However, after disabling hardware acceleration, the video seemed to speed up slightly. I tried disabling all of the add-ons and plugins to go through them one by one. Even with everything disabled except for Shockwave Flashplayer, the videos are still running slowly.

You can also try "Disable the hardware acceleration" in the Flash Player.

• http://helpx.adobe.com/flash-player/kb/video-playback-issues.html

See also:

• https://support.mozilla.org/kb/Cannot+view+full+screen+Flash+videos

Flash "Display settings" window:

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