Hi, this is pretty much the reason only Persona is now aloud : https://www.howtogeek.com/333230/why-firefox-had-to-kill-your-favorite-extension/ There are a few that when using the Background in 57+ such as : https://addons.mozilla.org/en-CA/firefox/themes/ some of these look great and can tap and test. As well as some stuff being tested if like loads of color : https://testpilot.firefox.com/experiments/color and old faith fulls like : https://addons.mozilla.org/en-CA/firefox/addon/theme-font-size-changer/ What is exciting is that Firefox can be customized in over 230 million ways with userChrome.css (Note : chrome has been in Firefox before there was a Google and a Chrome and stolen engineers) It is easy, know what you want but 1st need to read about 10mins to figure out : make a folder, make a file, put file in folder, where to move folder to and some copy/paste. So with that please see : *https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/ some info, some examples, some code : *https://www.userchrome.org/what-is-userchrome-css.html Where to find code is here if ask, though only a few know it or this dedicated sub reddit : *https://www.reddit.com/r/FirefoxCSS/ Seems most people are into just black and I have not seen creativeness as of yet. But if ask, might be able to help further. Most if not all Extensions have a comparable one for Quantum but will only find them if using Quantum. Hope that this was what you were looking for.

Extensions are gradually gaining more ability to theme different parts of Firefox, but you are correct that it is largely cosmetic. Extensions are not going to be able to modify the actual structure of the browser as they could before. To read blog posts about the new theme-related stuff in recent releases; Firefox 63 will have many more changes than Firefox 62 did: https://blog.mozilla.org/addons/category/personas/