Do you have the reload every like I do? If so, then you have it. Just have to enable it after choosing setting.
Maybe it is buried in an add-on then. I coulda swore it was just part of FF now, but maybe not. Tab Mix Plus is the only add-on that I could see changing the look of things that I have installed. I try not to run too many add-ons. https://addons.mozilla.org/en-US/firefox/addon/1122/