When you start the Firefox web browser, it will attempt to determine if the network connection available to it when it starts is via a captive portal. If you use a hotel's Wi-Fi service, a free WiFi service provided by a restaurant or other business, or some other public WiFi service, when you first open your browser you may see a web page asking you to accede to terms of service, an acceptable use policy, or to provide some authentication information, e.g., perhaps your name and room number for a hotel, or to provide payment information if you are accessing a wireless service that is not free. You won't be able to go elsewhere on the web, at least not easily, until you deal with the demands/conditions specified on the captive portal page.
Firefox makes a determination on whether there is a captive portal
constraint by attempting to download the file success.txt
http://detectportal.firefox.com/success.txt (there is only one word in
that file, the word "success". If it can successfully
retrieve that file, it can assume that it is not constrained by a
captive portal. You can see such attempts using the
specifying a filter of
"http.request.method == "GET".
[ More Info ]