ofcourse your demo works..... ( sigh )
I did "fix" the problem. I was using a link to submit my form that looked like:
<a href="BLOCKED SCRIPTdocument.forms[0].submit();"><img.. /></a>
That worked submitting the form, it just didn't post back the new editor HTML (in FireFox) as described above.
So I got rid of my <a> link and put in an <asp:imagebutton> to submit the form. It worked.
I don't know what the difference is, but it worked.