Since I'm studying at a university, I live in a room belonging to them, and to use the Internet, I have to sign in at their network with my computer, to which I'm directed automatically whenever I start Firefox.
When I started up today however, Firefox reported that the certificate of that sign-in site was somehow not ok, and asked if I wanted to add an exception (I guess that means accepting the certificate). I declined to answer, closed Firefox and restarted it again. The warning didn't reappear. I ran CCleaner, restarted my computer, and started Firefox again. No warning this time either.
So I wonder if that warning - that has only appeared once ever - is anything to worry about. How could the certificate be "temporarily" lacking? Do you think the fault was with Firefox or with that site?