Anyone having problems with video not working in Windows 8.1?  I finally fixed mine.
If you have any hardware from Nvidia then you may have a conflict.  Open Task Manager and end all tasks (processes) for Nvidia. Do not stop Services.  That should fix the problem!   I also run The 64bit beta version of Mozilla Firefox called Nightly.

Problem:  Videos do not play in Firefox after upgrade to Windows 8.1.

·         Solution:  Open task manager and end tasks for all running Nvidia processes.

This should resolve the issue. *Don’t confuse processes and Services.

 (Note: the processes stopped have no noticeable effect on my work or games).

             It should be noted that I also...

      Installed Nightly – Mozilla Firefox for 64 bit machines

·         Updated drivers

·         Installed K-Lite Codec pack

·         Installed Silverlight

·         Installed Flash Player

·         Installed Shockwave Player

·         Installed Quicktime

·         Installed Firefox plugin for .wmp files

      Windows 8.1 video Issue solved!