Hi Adam,
Now i find this is an issue of Firefox, and it was fixed in version 1.5
It relates to DesignMode of IFRAME
Now everything is OK with Firefox V1.5 but FF V1.0.+ and Netscape are not
I think we should show another IFRAME (always off) when switching to Preview mode

BTW, I find other (small ...) bugs about DesignMode when using your Editor
Thanks,
Bodom