I have tried everything, from the official latest release of flash plugin from Adobe website , to the flashplugin-nonfree of the repository (that it's the same version....10.0.22.87).
I use Kubuntu 9.04 64bit.
Firefox installed from repo, latest version 3.0.10.
When I am seeing a flash video in firefox sometimes the video flash become of grey colour and I have to reload the web page to see again the flash video (and of course to reload it again -.-)
I actually don't know if it's an issue related to firefox or to flash itself.
This is a common issue, many 64bit users have it! Flash is actually a necessary element of every computer, it's quite hard this problem.