Firefox doesn't send the referer header unless it's sending a POST request. Is you're token secured by referer or IP?
The tasks automatically switch to using a POST when a token is present. Can you use a tool like HttpFox to verify whether the requests are using POST or GET?
<mx:HTTPService id="gpService" fault="gpService_faultHandler(event)" method="POST" result="onJSONLoad(event)" resultFormat="text"/>