A while ago I noticed that Firefox began to have issues with loading the web interface for our Pfsense firewalls while other browsers loaded the website with no problems.
Here is a picture showing the timings of the web request for the index page
Then I found out that Firefox had changed the behaviour of how it handles SSL certificates in a recent update
There it states that a server certificate are not allowed to have the CA flag set:
Are not allowed to have basic constraints asserting isCA=TRUE.
And if you check the self-signed certificate used by default in pfsense it has the CA flag set.
So I created a CA certificate in Pfsense and then a "server" certificate signed by the seperate CA certificate and configured Pfsense to use the new server certificate instead. This change fixed the issue and now Firefox is loading the web interface without problems.