If the above ideas fail to resolve your problem, there are still a few things you can do to try to resolve the issue.
Update Firefox – This may seem obvious, but if you’re running an older version of Firefox, it’s probably best you update – not only to get any performance benefits, but to plug any known security holes. You can always find the latest version of Firefox on the Firefox web site.
If you notice the problem occurs when you are running a flash video, you may try updating the Adobe Flash player for Firefox. You can find this here.
Another possible issue is interference with another program running – one possible culprit is your anti-virus application. You may try disabling the real-time scanning to see if the issue still occurs. If disabling the scanner fixes the issue, check for an update with your antivirus vendor.
The last step would be to completely remove and reinstall Firefox. Be sure to start with a new profile as outlined above.
Hopefully one of these tips will help you out!