How to stop Firefox's version 64.0 from showing the update nag screen?

Solution 1:

Is there a way to stop the update nag screen?

Yes. Read on.

The option to Never check for updates was removed from about:preferences. You can use the DisableAppUpdate enterprise policy as a substitute.

Source Firefox 63.0, See All New Features, Updates and Fixes

Policies can be specified using the Group Policy templates on Windows (https://github.com/mozilla/policy-templates/tree/master/windows), configuration profiles on macOS (https://github.com/mozilla/policy-templates/tree/master/mac), or by creating a file called policies.json.

On Windows, create a directory called distribution where the EXE is located and place the file there.

On Mac, the file goes into Firefox.app/Contents/Resources/distribution.

On Linux, the file goes into firefox/distribution, where firefox is the installation directory for firefox, which varies by distribution.

...

DisableAppUpdate

This policy turns off application updates.

{
  "policies": {
    "DisableAppUpdate": true
  }
}

Source policy-templates/README.md at master · mozilla/policy-templates

So create a file called policies.json containing:

{
    "policies": {
    "DisableAppUpdate": true
    }
}

And place it in the appropriate directory as documented above.