After navigating away from YouTube, Firefox hangs and then shows video still.
Whenever I watch videos on YouTube, everything works fine. When I navigate away everything will be fine for a few minutes then all of a sudden Firefox locks up and shows just a still picture of the video I had watched. Not the page, just the square of the video. Then my whole PC locks up.
Wšykne wótegrona (1)
If you use extensions (Firefox/Tools > Add-ons > Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure that such extensions aren't blocking content.
Firefox isn't able to lock the computer, only programs or services that run at kernel level can cause this.
Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen) as a test to see if that helps.