I had a quick look at this yesterday.
In my installation Firefox is already set to use geoclue for location, whereas support for gpsd was deprecated some time ago, thus it seems to me that if you were to configure geoclue to utilise GPS data from your telephone via rfcomm/bluez then you might get this working. Could be a bit of faffing around but, depending upon the degree of need, may be worth it?
I'd have given it a try, out of interest, except I don't use an Android based 'phone (nor iOS!)...
FP.