I'm not disagreeing with anything you've said?
I'm saying that just adding Mozilla's PPA to your sources won't change apt's behavior when installing Firefox unless you tell apt to prefer the package offered by the Mozilla PPA.
As someone who uses Kubuntu as a daily driver, I'm well aware of the snap drama and have worked around it using the method I pasted above.
Even though it's an underhanded move by Cannonical, I'm still glad the OS is open source since it makes the workaround so trivial.
