I imagine the overlap of people who use Snapchat and people who use Firefox is pretty small, they probably see such a small amount of users with Firefox and they just decided not to support it.
So often just swapping the user agent from Firefox to Chrome makes these sites work flawlessly. So they're putting in extra code to detect Firefox and serve a "we don't support your browser" page when they could just... not. And if a user complains about X, they could say we don't test on Firefox, try on Chrome.
Snapchat has been a shit company for years. They threatened to sue third party client developers for Windows Phone, they purposely degrade camera quality on Android, etc (For awhile on Android they were just screenshotting the viewfinder instead of actually using the camera APIs.)