'connection is insecure' when starting Firefox from Thunderb

[dgulotta]

If I open Firefox by clicking on a link in Thunderbird, then whenever I try to visit an https site, I get a warning saying "Your connection is insecure". If I open Firefox normally, this does not happen. It does not matter whether I visited the https site by clicking on a link in Thunderbird or typing it into the address bar; it only matters how Firefox was first started. Any ideas on what could cause this?