Can't view videos

2 different problems playing videos :

1) When I first created a project i viewed the video, which was ok, but then i pressed F11 to go fullscreen so I could see all of it, then it dissapeared.

2) THen i went to the help section to view the video again, but this time the video played but i couldn't see it. I could see a bit of the player bar, but thats it.

I have the latest firefox and flash on linux.