I fixed it by clearing site data. Odd that it didn't show the button though.

If you have the Advanced button but can't make an exception and proceed, this could be due to the site having previously instructed Firefox to only allow secure connections. If you see it again, check for anything along those lines.

Also, if the site should be handling security properly, this error could indicate your connection being intercepted. If you want to compare the issuer information you can use the Page Info dialog (Ctrl+i), Security panel, View Certificate button to see the site certificate details.

The site is using a self-signed certificate, it is expected behaviour.