There's a simple reason why Mozilla/canonical does this and that is security fixes. Due to the difference in support cycles of Firefox and Ubuntu LTS versions fixes would have to be manually backported to the system Firefox version and newer versions won't run due to library dependencies. Snap solves all of that.
Don't get me wrong though, snap is still terrible, but other than flatpak or doing the work of backporting it's the only option to get security fixes to Ubuntu