Neither Adobe nor Mozilla have been able to completely fix the problem that has been plaguing some Firefox users ever since their Flash plugin got updated to Flash 11.3, the latest stable release which introduces Protected Mode, the Flash sandbox.
Both companies have alerted users about the issue and have provided some help documentation and workarounds, but no solid fix. The update to Firefox 13.0.1 fixes some of the problems but not all.
But at this time, there is no complete solution. With the mighty BBC writing
about this, you know the issue is big.
One way to fix the problems, which are mostly caused by other plugins or add-ons which clash with the latest Flash is to disable the plugin responsible, if you can find it.
As a last resort, you can revert to Flash 10.3 an older but still supported version. A more detailed explanation of the steps you can take to fix the problem is available here