I would recommend you to install Firefox ESR (extended support release) on those computers. The last update (version 60) includes a new feature: A policy engine, that should allow you to do just that (default settings for every client): https://support.mozilla.org/en-US/products/firefox-enterprise/policies-enterprise The policy engine also works together with the windows group policy The policy engine also allows you to disable profile refreshs or prevent users from accessing about:config and a lot more. I couldn't find a setting to disable layout customization, yet. I'm still searching. If Firefox doesn't have one, then I'll open a feature request.

We are looking into adding a policy for this.