There doesn't seem to be a concensus as to which shoulder; it varies among units of the military, and among civilian organizations. The main concerns seem to be keeping the stars toward the front, as if the flag were flying in the breeze, and that the American flag has higher position over other flags or emblems.
It would probably be best to find out which sleeve the other firefighters at the station are displaying American flags on, so your shirt fits in.
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Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them