Try these troubleshooting steps:

• Firefox uses too much memory or CPU resources - How to fix

Crashes or other problems with certain multimedia content in Firefox (such as Flash videos, animations or games) can often be resolved by performing the steps in these Knowledge Base articles:

• Flash Plugin - Keep it up to date and troubleshoot problems

• Adobe Flash plugin has crashed - Prevent it from happening again

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The failure of my adobe flash plug-in has never froze my PC. This type of failure would just kill the video. Also, the video was the only thing running at the time of freezing.

check this:

• see if there are updates for your graphics drive drivers

https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

https://forums.adobe.com/message/4468493#TemporaryWorkaround

• disable hardware acceleration in the Flash plugin

https://forums.adobe.com/thread/891337 See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

If the operating system crashes or hangs then this means that there is a problem with other Windows components like device drivers or services that are running.

Make sure that you have the latest updates of all components and that you security software in fully compatible with current Firefox releases.

I have Windows 7. I receive regular Windows updates. My firewall is ZoneAlarm, from whom I receive regular updates.