In opposite to many posters I want to activate automatic updates in Firefox. My Dell came with Ubuntu 18.04 and - surprise - instead of Firefox I got Chrome and Chromium. I installed Firefox (69) Snap. Auto update is disabled and I can read that “browser is maintained by system admin”. I am a retired private person so it is obviously me. (I am able to open open about:config or terminal, though.)
Firefox, like many modern applications has a built-in automatic update mechanism. However, when delivered as a snap, this mechanism isn’t used. The snap will automatically download and update soon after Mozilla publish a new version to the store. You don’t need to do anything further.