For normal Apt updates [0]

  sudo dpkg-reconfigure --priority=low unattended-upgrades

or edit /etc/apt/apt.conf.d/20auto-upgrades.

For Snap [1]

  snap refresh --hold=forever firefox

Or for all Snap applications

  snap refresh --hold=forever

(Various other useful options are documented, like only updating at weekends, or when the application isn't running.)

[0] https://help.ubuntu.com/community/AutomaticSecurityUpdates

[1] https://askubuntu.com/questions/930593/how-to-disable-autore...