I am having the same problem as the original poster. I am running Windows XP, the latest version of Firefox, and the latest version of Flash. I tried your test videos. Test 1 worked. Test 2 did not. I need to resolve this problem as fast as possible, so any information about what makes these videos different, and what I should do from here, would be greatly appreciated. Thanks!
Could you give our 11.4 beta release a try to see if that resolves the playback problem? If it does not, please see this FAQ for other possible solutions. In particular, look towards the bottom for instructions on rolling back versions or disabling protected mode.