While I don't know why it's being pinged that often (but I can guess), it's purpose is to provide location services when you are using websites (with the help of Google, surprise surprise). Since you've disabled GPS and blocked Firefox Nightly's location permission, it doesn't not have access to your location. Alternatively if it bothers you, you can gut it entirely by going into about:config and making the following changes:
geo.enabled = false geo.provider.network.url = blank (delete the entry)
If you change the first one you can reenable it later if you need location services for a website. The second change deletes Firefox's connection to their location services entirely.