If you have constructive ideas how the upgrade process can verify an arbitrary repository added by the end user, by all means, suggest it in the right place.
Upgrade process defaults to disabling any non-standard repository. How can upgrade process be responsible for anything user does after upgrade has completed?
What made you think this topic is about upgrade to start with? As usual with OP, there are some messages without any explanation.
None of my Leap installations has repositories from the mozilla project, still firefox gets installed and updated. What makes you think mozilla project is mandatory for every openSUSE installation and it is imposisble to update firefox without it?