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.

Was it referring to browser.xul? There’s no XUL left, so you might need to change that to browser.xhtml.