What is the purpose of GETting this external file?

It is used by Firefox when detecting whether it is using a captive portal.

A captive portal is a Web page that the user of a public-access network is obliged to view and interact with before access is granted.

Captive portals are typically used by business centers, airports, hotel lobbies, coffee shops, and other venues that offer free Wi-Fi hot spots for Internet users.

http://searchmobilecomputing.techtarget.com/definition/captive-portal

Captive portals are generally found at public wi-fi hot-sports, Starbucks, Airports.

Firefox uses this to detect if it's trapped in a captive portal (success.txt not found) or can really access the internet (success.txt found). In other words it's just Firefox checking for a proxied environment.

If you put that URL into your hosts file. Firefox may decide that you are caught in a fake wi-fi hotspot.

Source: https://groups.google.com/d/msg/mozilla.support.firefox/TJgV_uODg_I/uEe30_njBAAJ

To disable this:

1. Enter about:config into your location bar. Accept the risk.
2. Locate and set network.captive-portal-service to false

Source: https://www.blackhillsinfosec.com/towards-quieter-firefox/