Comment 7 for bug 1589076

Firefox 55 beta supports two new independent about:config settings, widget.chrome.allow-gtk-dark-theme and widget.content.allow-gtk-dark-theme (https://bugzilla.mozilla.org/show_bug.cgi?id=1158076). The former gets you a dark-themed UI without messing up web page content.

Official PPA for Firefox beta, if you don’t want to wait: https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-next