I open up a video fullscreen in the browser which works for about fifteen minutes until it freezes.
Can't escape out of fullscreen and moving the mouse doesn't move the cursor. Completely freezes the computer so I have to switch off the computer.
I was able to stream videos just fine for hours before this update occurred.
Happens with youtube, giantbomb.com and southparkstudios.com from the few I've tried
Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Once you get the pop-up, just select "'Start in Safe Mode"
When I found I could play streaming video in Chrome, I knew the problem was not Windows 8.1. It was something else.
I found the problem was I had two versions of flash. When doing the latest update, it did not disable/remove the old one. Hence, conflict. Hence video stopped playing. I disabled the older version. Now Flash works just fine.