Reminder that Firefox has a pathway to specifying some settings, including ones not exposed to users any other way, with a config file stored on disk.

They call it enterprise policies but anyone can use it by just putting a file in the location indicated on that site.

You can disable entire features, opt out of Telemetry before your first launch of Firefox on a new install, declare you never want to be part of studies, turn off their ML integration and keep it off, force about:config preferences in a way that can't be "accidentally" reverted, etc.