By convention, when this happens, it means that the site has either been blacklisted or without proven identity in the database of Firefox, so for these two cases, does the same rule apply ?
I think the culprit is Internet connection speed or the networking of the PC at the time it is running.
Take internet connection speed as an example. When the speed is too slow, the certificate will not be loaded successfully and hence Firefox decided against validity, and issued warning about possibility of untrusted site.
Just restart your PC and your Internet connection, it will come back working normally.