This is a Flash player + wmode=transparent + Firefox specific bug which is
independent of SWFObject. Unfortunately there is no known workaround for
this issue.
You can place an image as the background of your Flash file to match the
elements / layers you are trying to integrate with on the HTML page, but
that is about as good as it gets.
This post MAY help your specific situation (but may not)
http://www.quantumslip.com/archives/2006/09/flash_wmode_scr.html
Aran