This map uses arcgis jsapi 3.19, compare to leaflet js, it is quite heavy and demanding for low-end pc or mobile. jsapi 4.x is more lightweight though.
firefox is awesome but it allows you to screw up. you are probably running a bad extension that makes it that slow. if you have ad blocker, make sure it is ublock origin.
also, it is the only browser that allows extensions on mobile.
