Like button not displaying in Firefox

Hi all

I managed to add the like button quite easily to my website, it works like a charm in Google Chrome, Opera, Safari and IE9. When I open my site in Firefox, the wrapper div of the FB button is displayed, but the html within the iFrame is empty. Someone else experiencing the same behavior?

- Test page: http://mss.matthiasmuller.be/presentatiecoaching
- Switching XFBML to HTML5 didn't solve the problem.
- Updating from 7.x-2.0-beta4 to 7.x-2.0-dev didn't too.

Thanks in advance,

Matthias