I do NOT disagree with anything you (Nobbie) say and I do appreciate the feedback. However, I would point out that this problem is a bit peculiar to those of us using XAMPP (i.e., running personal/private servers on our own networks). I'd also point out that, I think, XAMPP (i.e., Apache Friends) are the creators/suppliers of the files we're discussing (i.e., server.crt, server.key, and server.csr).
Also, I've been unable to find any pertinent information for using self signed certificates by searching the web. However, I have been able to find articles regarding the procurement and installation of real trusted certificates obtained from recognized Certificate Authorities. Why do that on a private network unless self signed certificates are NOT possible?
My investigation has also lead me to learn something else that might be relevant. With a little hacking around I found an item referred to as
View Certificates in the Firefox settings. Selecting that takes you to something labeled the
Certificate Manager which has capability to import certificates. When I tried importing the file named "server.crt" the operation completed without any error indication. This included an opportunity to view the imported certificate. A printout of that screen is
shown here. Something that caught my attention is where it says Validity, Not After, Fri, 08 Nov 2019 23:48:47 GMT. Also, after completing the import and restarting Firefox the imported certificate is nowhere to be found.
This raises the question, "are the self signed certificates supplied with XAMPP even usable?".
You are also quite correct when observing that technology (i.e., including browsers) needs to perpetually change in order to stay relevant.