If you have installed a certificate into your personal certificate store, annoyingly Firefox doesn’t read from that certificate store. Firefox somehow think they know better to trust the certificates that you have told your computer to trust and how certificate services work throughout the rest of your computer 🙁
To fix this, you have to open Firefox, click on Options and find the Certificates button, then import the certificate using the Firefox interface.
I have no idea how to do this on mass with Group Policy. I would assume that there’s a Firefox GPO that you can import to manage certificates in Firefox. If anyone knows, please share the answer below!