That's a slightly worrying precedent, they're calling the way to diaable certain bits of obnoxiousness (the header bar in Tree Style Tabs comes to mind) "legacy", which is usually a precursor to removal.
Is there a new method available/forthcoming, or is this more control planned to be wrenched out of user's hands?
Was set to true already.
Set this option to true: toolkit.legacyUserProfileCustomizations.stylesheets