If it doesn't happen in Chrome or Internet Explorer, you know what browser to use for those sites.
Firefox always gets unresponsive when I browse onto video intensive websites. And give me this message.
Warning: unresponsive script
A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the script in the debugger, or let the script continue.
What to do? I'm on window 10, flash player and everything is up to date, I'm using the original video card that comes with the PC.