Turns out this was an issue with Firefox's automatic certificate selection logic. I changed it to ask me every time instead of automatically selecting certs, and once it saves my selection it's back to normal.
I'm not sure how that changed in this release, but it's at least resolvable.
Does FF use a different cert-store than os? I've frequently had to fiddle with os/app cert-stores to get them to trust. Not just import the cert but the signer too, some times on both sides. I'll likely hit this issue when I upgrade.
It does have it's own cert store. You can test your sites with testssl.sh [1] to see if they validate correctly. It only depends on openssl and bash. If you have your own self signed CA/certs, then you would have to import them into FF.
