I have this problem too, with an nvidia card. The ghost images
show up in almost every type of window. To stop it I run this
tiny script (called kp) to kill the flash plugin:
#!/bin/bash
killall plugin-container
The next time flash is needed, plugin-container and flash are
automatically regenerated (you may need to refresh the web page
in Firefox).
BTW, this Firefox add-on, Video DownloadHelper, is great for
downloading videos from Youtube and other video sites.
https://addons.mozilla.org/en-US/firefox/addon/video-downloadhelper/
You can then watch the videos with mplayer, and get brightness,
contrast, speed, color, and other controls -- much better than
what you get with flash. Also, if the video is important, you
can save it; anything on the Web can go away.