I have been on this all day only to find out AdBlock plus extension was filtering my request URLs!
When I updated the subscription the issue was solved.
[Exception... "Component returned failure code: 0x805e000a [nsIXMLHttpRequest.open]" nsresult: "0x805e000a ()" location: "JS frame :: http://localhost:8000/xhrtest.shtml :: makeRequest :: line 35" data: no]
This URL succeeded: http://localhost/cgi-bin/resource/partner/one
This URL threw the exception: http://localhost/cgi-bin/resource/partners/one
The exception is thrown when open is called on an XMLHttpRequest object.
The AdBlock plus subscription was filtering on this pattern: */partners/* !
Yeah pretty general pattern but updating the subscription improved the pattern.
It looked pretty bad for a moment there and I reflected that this was the first time I saw a bug in FF when MSIE was still working ! I found the problem when I started following the standard FF bug filing report rules: just goes to show - it pays to follow the rules
