I just figured a way
Firebase.enableLogging(function(logMessage) {
if(logMessage && logMessage.indexOf("Long-poll script failed to load") !== -1 && navigator.onLine){
console.error('Its dead Jim! Firebase seems to be down for this user');
}
});
I'm not sure how firebase works under the hoods but I've seen users connected in a previously open tab and if they open a new tab firebase won't connect when it's down.
This logging function is called a lot, and when firebase is down our console.error will be call every ~5s or something.
Make sure to debounce whatever you'll do in the console.error's place.