i thought i had solved this by adding the header for accept-ranges - since when i did that i could then seek in firefox on windows 7. however, after testing further i found that the results are inconsistent and ultimately it is still somewhat broken.
i have added more details to this to a new question i have asked at serverfault: http://serverfault.com/questions/710304/why-is-partial-content-not-being-served-in-nginx-mp4

any input is still welcome!