They haven't modified apt; they abuse an extra version number that supercedes the major version number of a package. I think it's meant to be used for new packages that reuse the name of an abandoned project. Canonical publish packages for software like Firefox that depend on snapd and just run snap install firefox instead of actually installing anything. Since they bumped that extra version number, their packages always have a higher precedence than even the officially packaged debs from Mozilla.
If your filesystem is btrfs then use btdu. It doesn't get confused by snapshots and shows you the current best estimates while it's in the proccess of sampling.
Yup. Even if you add the official mozilla repos, Cannoical adds a prefix to their version so it always takes precedence over the official release. You have to pin the mozilla repo to blacklist the snapped version.
Same goes for Thunderbird.
I'm sure Snap has some security advantages for many users but they've made it so user-hostile for those who use native browser extensions or who want to automate deployments with just one packaging system.
Only if they're travelling at relativistic speeds.