The browser won't open any https:// pages anymore

From one moment to the next, FireFox won't open any SSL-secured (https://) pages, while it worked perfectly a minute ago.

This problem is known for some versions of FireFox and happens when a previously opened certificate properties window is still open (often on the background). Firefox refuses to open any new SSL-secured pages when this is the case. You can solve this issue by closing the certificate properties window, and by updating to the latest release of FireFox.