Here's a wierd one. Let's say you have a flash navigation - one of those old DHTML style drop-down ones, but done in flash and layered over the top of your page content. You can stick the wmode to transparent and have a transparent background for your flash.
The solution I found has prettymuch baffled me as to why it should work, but it does. Using the fully-qualified class name for ExternalInterface (
flash.external.ExternalInterface) solved all of my Firefox woes.
So quite simply - change your