I am connected to my own router, and I know for a fact that I do not have a captive portal installed. Firefox displays a banner at the top of the page saying that "you must log in to this network before you can access the Internet.", with a button that says "Open Network Login Page" to its right.
The link forwards to http://detectportal.firefox.com/success.txt. Notably only one machine is affected. Just sort of wondering what the possible reasons are for getting suddenly seeing this.
I also know that someone hasn't set up a fake router SSID- because I can see that I'm connected to the router via the router's web-config utility