As a side note, know that your naming is typically against convention. Generally, variables prefixed by m are private member variables, accessed & mutated by public methods (like yours).
For your case, you might just be able to change your methods to be getMLat() and setMLat(). Although I'd suggest changing up your conventions a bit in general.
I had the same issue just now & the answer was pretty frustrating.
I think that the issue is with your naming convention. For example, it looks like you named a variable mLat, and then your accessors/mutators are getmLat() and setmLat(). I think when Firebase is doing the deserialization, they rely on certain naming. For example, if you have a member variable named lat, you will need to have getLat() and setLat().