Thank you for the comments,
I've forgotten to mention; the OS is 64bit Ubuntu 10.10.
I saw, firefox was 64bits. And when I
(note:I write this from memory, may not be correct)
I saw the flash player being currently used was 32bits.
It must have been about the inconsistency between 64bits firefox and 32bits flash.
I have removed the current Flash Player using Synaptic Package Manager and installed Flash 11 64bits using the directions here
After I restarted Firefox, it seems Flash is working perfectly now, that is no image residue on the desktop or so.
Thank you for the guidance.