"obj has no properties" in Firefox

I'm having problems including the FCK... In the development area (no frames, no securing, just the intranet), i'm having no problems at all using the FCK in Firefox and IE 6 or 7. But in the live area i get the error "obj has no properties". In the live area my php tool is put into frames. It's an extranet where you can login to our intranet using sessions.. In IE 6 or 7 there are no problems, but when i'm using it in Firefox, i'm getting the toolbars, but no textarea...screenshot below..

Anyone who has an idea? When you need more info, just let me know..

Thx in advance