This is nice to know but in future versions of Firefox that single config switch (browser.ml.enable) will both change names and split into multiple sub-switches, most likely appearing in different pages of about:config.
These sub-switches will then not remain consistent.
Bank on it.
Almost no one changes defaults, according to the experts
It's perverse to think about:config is doing favours for anyone except Mozilla and its business partners
Otherwise the defaults would be false, i.e., opt-in