What screen saver is it -- the one built in to Vista?

Are the "problem" videos native HTML5 videos, or videos playing in a plugin such as Flash or Silverlight? You usually can tell by right-clicking the video and checking the menu (i.e., regular Firefox context menu or plugin-specific context menu).

yes the one built into Vista. Im having this while watching YouTube videos and also from other websites. After right-clicking the video a menu appears:

Settings... Global settings... About Adobe flash video player 11.9.900....

hope this helps

I have just found out that this glitch has also affected my video playback in my VLC player which was absolutely fine before installing firefox. Please help because I don´t wanna go back to Internet Explorer or Chrome.

Cheers

The Flash Player plugin normally should block the system from going to sleep, reducing display brightness, or going to screen saver when you use its full screen mode. It's strange that you have this issue with Flash.

There are some workarounds, like using an add-on or external program that simulates moving your mouse, but those really shouldn't be necessary.

VLC, I don't know.

By the way, have you shut down and restarted Windows since this problem first occurred? Just in case some shared component of Windows has crashed.