I'm still not clear as to why when Mozilla does something different then the others,
Mozilla is correct and all the others are wrong.
You have to have been following it from when it first occurred to have an idea of what happened. The whole idea of HTTPS is to prevent snooping on encryption of data between the user and the site they are visiting. AVs want to have a position as the MITM (Man In The Middle) to intercept and scan that traffic.
So it is pretty essential that Security software companies understand how browsers prevent these MITM type interceptions so they don't get broken.
Mozilla had previously made it clear what the AV companies were doing wrong which had the effect of breaking access to HTTPS sites because the Security Certificate wasn't correctly placed. This by all accounts is still happening in some cases. Hence the requirement to do something to manage this more graciously, not to penalise the end user, who are more likely to blame the browser rather than their own antivirus program.
This 'problem' isn't evident in most of the other browsers so why are they able to function without any problems?
I can't speak for other browsers as I don't use them.
I also didn't have a problem on my XP Pro system as:
1. It uses Avast 18.5.xxxx
2. It uses an old version of Firefox ESR 52.9.0 (32-bit).
I also didn't have a problem that I can recall on my win10 system, but I believe I made a change the the firefox about:config settings.
But it is most certainly Avasts task to ensure that it does work, in much the same way as it has to with the AOS firefox add-on.