Firefox is no longer supporting disabling the auto update feature via about:config.
Previously there was a setting app-update.enabled which could be set via a vendor.js file.
This was moved to the policy engine in Bug 1420514.
Since I'm managing Firefox Nightly with my package manager I wanted to disable auto update again.
In order to do this simply create a directory called distribution in within you Firefox installation directory.
Then create a file named policies.json with the following content: