I'm experiencing the same problem as in the original post: the tv.adobe.com instructional videos present only a black rectangle. I'm running OS 10.6.8, Firefox 26.0 (in either normal or safe mode -- makes no difference), and the latest version of Flash (14.0.0.125).
I see almost the same problem in Safari (5.1.8), except that in Safari there's a link for the HTML5 alternative, which works. In Firefox, that link is not visible.
The Settings and Global settings (accessed by right-clicking the video window) seem correct; hardware acceleration and local storage are both enabled. What other settings might be relevant?
Thanks for any insights -- this is frustrating.