Firefox Detectportal GET success.txt
Today I was looking at some Flask-WTForms pcaps to see how they worked when I noticed something weird. Every second or so, Firefox sends a GET request to http://detectportal.firefox.com/success.txt.
After a bit of reading I found out it is used to check whether a client is in a captive portal. These are wi-fi hot-spots which have a web page on which the user has to login, accept a terms of service/EULA or something similar, like for example on Dutch trains: