A last bit of research leads to http://code.google.com/p/fbug/issues/detail?id=986&q=Permission%20denied
. This seems to suggest that it's a bug in firebug blocked by a bug in firefox. I did note along the way that even in my original profile, disabling firebug removes the symptom. Why getting a new profile helps is a bit of a mystery but there is doubtless something within firebug that gets reset.
All that being said, the code "n && 3 == n.nodeType" is of the sort mentioned in the previous two references and I'm inclined to agree with Edwards' remarks.
Thanks again for your interest and help.
> As an interesting post script I found this
> fcafb18?pli=1> concerning jQuery. It references an interesting note
> here <http://dean.edwards.name/weblog/2007/08/firebug-slow/> . Perhaps
> this is related.