I sniffed with TCPDUMP in my FreeBSD netbook what's going on on
first requesting with firefox some web page:
FF starts and asks the (local) DNS at IP 192.168.179.1 port 53
for the IP addr of A? detectportal.firefox.com
DNS answer: CNAME detectportal.firefox.com.edgesuite.net.,
CNAME a1089.d.akamai.net., A 92.226.1.104
FF makes HTTP request to 92.226.1.104 and sends
HTTP: GET /success.txt
92.226.1.104 (and this is perhaps not the answer of 92.226.1.104, but sent by the local AP):
HTTP/1.0 302 Moved Temporarily
Location: http://192.168.179.1:8186/untrusted_guest.lua.
which FF now let bring up the landing/login page of the AP
But, this (HTTP redirect) is only one of more arts to direct a browser to
a login page; see also: https://en.wikipedia.org/wiki/Captive_portal
I also would suggest a good organized description in the Wiki about some common methods to try in UT/UB to get to the login page of the local Wifi AP.
matthias