Firefox won't connect to git.ipfire.org

Started getting invalid certificate errors from Firefox when accessing any https pages on git.ipfire.org. It was OK earlier and I can still access from chrome(ium).

Firefox Quantum 64.0

"Your connection is not secure

The owner of git.ipfire.org has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.

This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox may only connect to it securely. As a result, it is not possible to add an exception for this certificate."

Rob