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Another theory is malware in an addon injecting the packets coming back. Most likely not though since you are modifiying the headers. As of the Addons, the problem was reproduced with several clean installations of Firefox, with no addons and even on safe-mode, so I would say that the addons might not be related.
Regarding the proxy or local router, that might be involved, however there's the wireshark capture indicating that both, the good image as the bad one, came in identical packages. Is there a way to set the debug level of firefox to log everything that is possible to it.