It seems to have been a transient problem, as it has stopped happening now, but it happened for three new pages a minute ago. Clicking the link took me to http://detectportal.firefox.com/ which displayed "success" in plain text. Since the problem seems to have stopped, there's nothing to fix, but I think that it might be indicative of a problem at Mozilla HQ, or somewhere in between, on the interwebs. Thought that you should know.