I was surfing on Stack Exchange websites when I suddenly got the error message from Firefox about "Identity of this page could not verified" on all stackexchange sites. I had to confirm that I know that the certificate of these pages are unknown. Should I worry about that (and yes, I know that I am currently asking that on an unsafe Stack Exchange site)? If yes, what has happened?
All the other SSL pages (e.g. Google, my mail provider etc.) worked.