https://support.mozilla.org/en-US/kb/customizing-firefox-using-policiesjson

Taking macOS as an example, create a distribution folder in the Firefox.app/Contents/Resources directory. Then, create a file named policies.json in the distribution folder with the following content:

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{
  "policies": {
    "DisableAppUpdate": true,
    "ManualAppUpdateOnly": true
  }
}

Restart the browser and you will no longer have annoying update reminders.