It's due to the "unattended upgrades" feature. You can blacklist FF from getting those upgrades with a change to /etc/apt/apt.conf.d/50unattended-upgrades. Add

It's due to the "unattended upgrades" feature. You can blacklist FF from getting those upgrades with a change to /etc/apt/apt.conf.d/50unattended-upgrades. Add "firefox" to the "Unattended-Upgrade::Package-Blacklist" section. I find manually upgrading much less annoying than having my browser randomly lock up on me.