Finally, Firefox can be configured for addons. See https://github.com/mozilla/policy-templates/blob/master/README.md for how to do this and https://github.com/serv-inc/blacklist/blob/master/example_policies.json for basic usage. (This blocks deactivation and removal, at least on a fresh FF install).

---

Thanks for the suggestion. To disable all addon removal, have a look at https://addons.mozilla.org/en-US/firefox/addon/block-about-addons/. (If you install this, there is still one way to disable all addons at once, but that seems impossible to prevent. Firefox 60 and ESR 62 also provide some security: https://blog.mozilla.org/futurereleases/2018/01/11/announcing-esr60-policy-engine)