Flash from one tab visible in other tabs?

Hi all,

I got a new 64 bit PC and installed Ubuntu 10.04.3 with all updates.

Flash is
Adobe Flash Player
Shockwave Flash 10.3 r183

Problem with Firefox 6 and 7 is that when there is flash in a page it is also "bleeds through" (is visible) to other tabs making them unusable. The flash e.g. a youtube video even is visible in the background of other programs.

Occasionally the whole PC freezes for 10 seconds.

Any ideas what this could be?

Not sure if this is even relevant:

Video card is Nvidia GeForce GTX 550 TI and driver is NVIDIA 280.13.

System -> Administration -> Hardware Drivers shows:
NVIDIA accelerated graphics drivers (current version)
This driver is activated an currently in use.


Thanks.

Hardware is:
AMD Phenom II x6, 16GB RAM, Nvidia GeForce GTX 550 TI,