Youtube videos overlap in other firefox tabs

Everytime I display a video on youtube, and change to another tab, a snapshot of the video I was seeing keeps there. If i display several videos at the same time, pausing them or not, i see snapshot of all of them in all the tabs. I'm using a 64 bits Kubuntu 10.10 with Firefox 3.6.15.

http://img695.imageshack.us/i/imagezja.png/

Thanks a lot.