If you are using Firefox on Apple OS X and you are experiencing problems viewing the Flash content (you cannot see the Flash embedded content, you can see something only moving the mouse pointer over it, sometimes…), you must have the Adblock Plus add-ons installed.
This is a not very well known bug of Adblock on OSX. To fix it, you have to disable the option “Show tabs on Flash and Java” from the Adblock preferences menu.
Happy firefoxing folks! 🙂