I too wanted the same just write a small UserScript or just a ublock scriplet to modify viewport : function addViewport() { var metaTag = document.createElement('meta'); metaTag.name = "viewport"; metaTag.content = "width=device-width, initial-scale=0.45, maximum-scale=0.8, user-scalable=1"; document.querySelector('head').appendChild(metaTag); } document.addEventListener('DOMContentLoaded', function () { addViewport(); });

it's not a bug (I'm a long time user and I compile fenix from source )a few about:pref are "locked " like ipc.processcount