The Snap is by Ubuntu (and presumably will still be the way Firefox is installed by default on Ubuntu). I think that this is for people who'd prefer not to use the Snap, allowing them to install the .deb directly from the source.
I don't think that PPA was by Mozilla. I believe it was by Ubuntu contributors who volunteered to package Mozilla software (and possibly even the people who used to package the Deb before it was turned into a Snap installer).