The current version doesn't work with Firefox. It's getting fixed in a new update soon.
In the meanwhile, you can play using Chrome instead.
The update will fix many issues, be downloadable and add in 2 new Pokémon (Not revealed yet)
PawPunch is targeting mid-December for a release.
You can track the progress on their Discord server.
For the record. I am not part of the PawPunch team developing the game, but I am a moderator and I do follow the project closely.