I'm on a Mac and use mainly Firefox 8, but the problem that I'll describe happens with Chrome and Safari as well.
Basically, I can play most Flash content, including ads, games and videos, but there are some specific videos that do not play at all (the flash portion of the page stays black). For an example of such video, see this page (brazilian news site).
I have the latest version of Flash player and I was able to play the video using the URL of the player concatenated with the flashVars that I got from the source code os the page.
Any ideas on what's causing this? Thanks.