This pesky little problem has been niggling away at me for months, so I finally decided to investigate the problem and find a solution.

It appears as though this is only a problem in Firefox 3+ when you set the 'wmode' parameter to transparent. Now I'm no browser compatibility expert so I'll take someone else's word on this but Firefox renders and handles the default style of the outline property of the object tag differently to other browsers.

Solution

Set the object tag's outline property to 'none'

object { outline:none; }