My IT firm uses a lot of self-signed and built-in certs for in-house work and this new feature of Firefox has caused our Director of IT to decide that we shouldn't endorse the use of Firefox since half our sites would throw this "error" page. I understand what Mozilla is trying to combat but I think the author is correct when they point out that most people who would be tricked by a man-in-the-middle attack would be too scared or confused to understand that they can bypass the error page. I have added exceptions for some of these sites on my own network and it's an annoyance for me, but it would be impossible for some users to add the exception, at least without contacting me for help first.