@Herve5 In the release notes of Firefox for Android 35, it is stated that you can activate location scanning for Mozilla’s WiFi database.
@CDG I think the first step in your case is to deactivate EPO at all. (Settings > Location access > GPS Satellites and untick EPO). I read somewhere over here that EPO somehow is blocking A-GPS. And if you don’t mind Google having your data, you can also tick the option “Location via WiFi and network”.
If you want to give your data to e.g. Apple or another location database, you could have a look at UnifiedNlp.