Interestingly, it lets me use the mobile webapp messaging without asking me to do anything. Because the OS I'm on has no Messenger app, and it probably will never have one. So if you spoof your UA string, it may stop bugging you and let you use messenger as usual.
Parent is almost, but not quite, entirely useless. Would you mind sharing with the class what that particular UA is?
Mozilla/5.0 (Mobile; rv:47.0) Gecko/47.0 Firefox/47.0
I don't think its useless. Pretty sure Facebook didn't special case Firefox OS, they special cased android/ios. A random UA string (anything that doesn't say android) would be enough.