Can't use Firefox innerHTML() on XHTML page?

Hi,

I've written a very simple bookmarklet, which uses innerHTML to create
new DOM elements on a webpage:

(function(){
    var testDiv = document.createElement( 'div' );
    testDiv.innerHTML = "<p>I don't display</p>";
    document.body.appendChild(testDiv);
})()

For easy copying into a bookmark:

javascript:(function(){var%20testDiv=document.createElement
('div');testDiv.innerHTML="<p>I%20don't%20display</
p>";document.body.appendChild(testDiv);})()

Unfortunately, in Firefox 3.0.8, this bookmarklet blows up when
applied on this XHTML webpage: http://direct.tesco.com/q/R.200-3522.aspx

The Firebug error is:

uncaught exception: [Exception... "Component returned failure code:
0x80004003 (NS_ERROR_INVALID_POINTER)
[nsIDOMNSHTMLElement.innerHTML]"  nsresult: "0x80004003
(NS_ERROR_INVALID_POINTER)"  location: "JS frame :: javascript:
(function(){var%20testDiv=document.createElement
('div');testDiv.innerHTML="<p>I%20don't%20display</
p>";document.body.appendChild(testDiv);})() :: anonymous :: line 1"
data: no]

Can anyone explain why this doesn't work?

I initially thought that the Firefox innerHTML function couldn't be
executed when rendering an xhtml+xml document in standards compliance
mode. However, I successfully ran this bookmarklet against an xhtml
test document rendered from my webserver with a reponse header of
'application/xhtml+xml; charset=utf-8'.

This bookmarklet does not blow up for Safari 3.2.1 and Chrome
1.0.154.53

Any suggestions would be much appreciated.

Greg