Squid-Log-detectportal.firefox.com

I noticed in my logs that my Linux Firefox keeps making these requests

http://detectportal.firefox.com/success.txt

Firefox is apparently checking to make sure it is not behind a captive portal. Sinister? Probably just relates to spamvertising?

You can turn it off in FireFox
Open Firefox
about:config
set the network.captive-portal-service.enabled pref to false.