Since when this became a known thing? I’m aware that the snap version is installed when the user is trying to install the deb version of Firefox by running,

sudo apt install firefox

But I never heard that the installed DEB version of Firefox is replaced by Snap version of Firefox.

I’m aware that when the user runs(without adding Mozilla’s apt repository),

sudo apt install firefox

the snap version of Firefox is installed. But I never heard that, though APT is configured to install Firefox from Mozilla’s repository, the DEB version will be uninstalled and the Snap version will be installed.