No full video screen:You get a big blank white screen when you try to enable the full video icon. I finally found the answer-I just had to ask the question right for Mozilla's knowledge base to cough it up:

Here's the answer: Disable hardware acceleration

Here's how you do it:

Make sure you are using the latest version of Flash. To check your version and to upgrade, see Adobe's instructions. Navigate to a page that shows a video that you can't view in full screen. Right-click on the video player and click Settings-in the context menu. The Adobe Flash Player Settings screen will open. Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel. Remove the check mark from Enable hardware acceleration. Click Close to close the Adobe Flash Player Settings Window.

For the record, here is the Knowledge Base article: Flash videos won't play full screen

Hi: I had the same issue with white full screen but no more thanks to the talented person that post the help. Go on this page to "cannot view full screen flash videos" and follow the instructions or go on the video to watch and do right click, go to setting and uncheck the enable hardware acceleration, close, go the video to watch run it and click on full screen, enjoy