Firefox only updates on its own if installed outside of the package manager. This applies to Debian and its forks. If I click on Help -> About it says, "Updates disabled by your organization". I personally would like to see distributions suggest installing Betterfox [1] or Arkenfox [2] to tighten up Firefox a bit.

[1] - https://github.com/yokoffing/Betterfox

[2] - https://github.com/arkenfox/user.js