Did you compare the request byte-by-byte to verify that it's identical? I'd try that first. (I'm not familiar with firebug, so I don't know how to view the results as a hex dump. I normally use something like Ethereal to capture the traffic...)

If the request is *identical* and you're not getting the correct result, there might be some session traffic (cookie or some such?) that needs to be handled. I'm not terribly experienced with HTTP traffic, so some other monk will have to chime in on that, if it's relevant.