I’ve just install the last beta version of firefox and I can’t view flash videos in Google Reader. The flash player is working good on other sites like youtube, vimeo but not in Google Reader. I’ve reinstalled the flash player plugin and the problem is not solved.
So … the solution is actually much simpler. You have to disable the “hardware acceleration” option.
Go to Tools > Options > Advanced > General > and disable “Use hardware acceleration when available”.
UPDATE: The new version Firefox4b11 solve this problem. You don’t have to disable that option.