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();
});
yandex Browser which also support every Chrome desktop extensions(looking at you ublock ...) on Android has this feature glad to hear I'm not the only one I use it.
give nighlty I try on my second old phone were Firefox was not usable is now working quite well (for the old hardware snapdragon 660 and 4 giga of ram )the only sad part is opening or focusing url bar (which still show home )is painfully laggy and slow
happened to me too more than twice ...