https://www.techradar.com/news/make-firefox-your-own-with-two-new-experimental-features-color-and-side-view

Mozilla has released two new experiments as part of the Firefox Test Pilot program: Color and Sidebar. 

Most browser interfaces use subtle colors that fade into the background, letting you focus on the webpage. However, more choice is always welcome and the Firefox Color experiment gives you just that, letting you customize the whole window so it's as discreet or loud as you like.