Uninstalled Firefox still here
The page Removing Firefox in Ubuntu with all add-ons like it never existed addresses Firefox installed using APT but in your case, it has been installed as a snap.
This can be checked using the command type firefox. If you get an output like /snap/bin/firefox, Firefox is installed using snap on your machine.
To uninstall Firefox which has been installed using snap, use the command:
sudo snap remove firefox