You could use add-ons to manage optional functionality a la Atom. Users can enable and disable add-ons to customize their browser and some come enabled by default. If you were migrating to this method of customization it would absolutely make sense to push an enabled add-on that replaces functionality you took out of the main app.
Because this is not the scenario that is being denounced here. A brand new add-on with functionality unrelated to the product is being installed without consent or even notification - that's capital M Malware peddling.
Is this a thing ? History shows mozilla removing functionality but never replacing it. And then it's up to volunteers to make an extension to fill the gap, until mozilla breaks the extension or drops the extensions engine altogether.
