The company Mozilla is not doing any form of prizes or giveaways so I am not sure what you are referring to?

Did you get a site claiming you as a Firefox user won a iPhone or Android phone or a $1000 Amazon gift card where you pay just pay one or two dollars shipping for it. If so then it was obviously fake and just a way for the scammers to get people's personal information and credit card details. This scam has been popping up for several years at least.