If you don't like the redesign Mozilla has done with the new tab page and want to avoid the sponsored tiles,
...then switch to SeaMonkey
Because that's its accuracy for knowing your absolute position in a short period of time. If you use it to determine relative position over a long period of time, it's much more accurate. Apparently there's something called "Carrier phase tracking" which has an accuracy of 2mm for surveying. Or they could have augmented it with ground-based transmitters.
Still, 4mm is quite a small distance to measure with GPS, even with a 2mm accuracy mode.