This is a problem with the Adobe flash plugin for firefox which has proved difficult to pin down.
It will go away if you remove the plugin - but then the only video that will play will be theora embedded by the <video /> tag.
I have never had it, but then I usually block flash content on websites ... they are usually ads. The occasional flash I do watch does not trigger this so you may want to try noscript etc to control how much flash you see.
Some people have found it occurs after restarting firefox. Severity varies a lot.
Many people find that it goes away if they install nspluginwrapper. I cannot advise best practise for you to do this because I don't know your distro. Please edit your profile
to include this information. Thanx.