Browser reports connection to pfsense interface not secure
Where there would normally be a green lock with the word 'secure' just to the right, is a red triangle with an exclamation point and the words not secure to the right. HTTPS is crossed out. It did now want me to connect, but I did so anyhow. Do I need to add a certificate or something? How do I go about doing that?
To get a green icon, the site needs to have it's certificate signed by a Certificate Authority that your browser trusts.
pfSense uses a self-signed certificate, which means it's not signed by a someone your browser trusts by default.
You may want to read up on the finer details of HTTPS/SSL/TLS.
And once you're done studying up on that, check out the ACME Package so you can easily get a free trusted certificate for your firewall: