This happens in the aol browser and IE too.
I'm using Windows 7 Pro and cruising w Firefox. When most online videos play, sometimes there's no sound or I want to start again from the beginning, or pause it. When I move the mouse in the video, sometimes the Pause/Start, volume ctl, etc. will show up. When I move to the volume or pause icon - they disappear! This is plain weird. Any clues?