UI customization is fortunately still alive in the wake of XUL extensions. A /r/firefoxcss mod has a wonderful collection of code snippets that they maintain,

UI customization is fortunately still alive in the wake of XUL extensions. A /r/firefoxcss mod has a wonderful collection of code snippets that they maintain, which you can browse here: https://mrotherguy.github.io/firefox-csshacks/ https://mrotherguy.github.io/firefox-csshacks/ and they created a userChrome.js loader here: https://github.com/MrOtherGuy/fx-autoconfig https://github.com/MrOtherGuy/fx-autoconfig My favorite customization repository is https://github.com/aminomancer/uc.css.js https://github.com/aminomancer/uc.css.js - which really tests the limits of what is and isn't possible with userChrome.css and .js. My favorite feature is the implementation vertical tabs, without the use of extensions. Some legacy extensions are maintained and can be found here: https://github.com/xiaoxiaoflood/firefox-scripts/tree/master/extensions https://github.com/xiaoxiaoflood/firefox-scripts/tree/master... (although you will need to use xiaoxiaoflood's userChrome.js loader AFAIK). Honorable mention goes to the Firefox CSS Store, which can be found here: https://trickypr.github.io/FirefoxCSS-Store.github.io/ https://trickypr.github.io/FirefoxCSS-Store.github.io/ Moving to WebExtensions was the logical choice for Firefox, technical/security reasons aside, as they are not alienating extension developers that target Chromium-based browsers. Yes, they alienated their own extension developers. Yes, they could've handled the transition better, and worked harder towards supporting some of the many APIs/functionalities that extension developers needed (or still need) for their extensions to work in the WebExtensions ecosystem. I myself was quite mad for a very long time at how they handled the switch, but I think overall it's been a success - my own personal feelings aside.