How do you completely turn off update checks in Firefox v64? You can use a policies.json file to disable updates for all users. https://github.com/mozilla/policy-templates/blob/master/README.md or https://winaero.com/blog/disable-updates-firefox-63-above/

Thanks but I don't want to disable automatic updates. I just want to be able to defer their installations. Let's say my computers are running version 72.0.1 for few weeks and today version 73.0 is released but is not yet comptatible with a third party extension because of a new way to handle them. I'd like to be able to defer the installation of version 73.0 for 15 days for example. It will then give time to the developers of this extension to make the proper changes and release a new version.

There is no way to 'delay' the updates. But using the above, you can choose when to allow it by manually updating.

You can use a policies.json file in the distribution folder in the Firefox program folder to disable or enable updating.

Updates disabled:

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

Once you are ready to update then you can edit the policies.json file and enable updating. { "policies": { "DisableAppUpdate": false } }

Thanks to both of you. Failing a satisfactory solution (a feature developped by Mozilla) for business environment with thousands of computers, a workable accommodation seems to lie in switching to ESR and dealing with policies.json to delay (disable then enable) major updates.

Update : In fact switching to ESR should be enough because a major update is only installed when the .2 minor version is released according to the following kb. For example ESR 68 will be updated to ESR 78 when ESR 78.2 will be available which will happen 12 weeks after.