This is a custom userchrome file for Firefox (a file that is found in firefox installation that controls the UI) with the main goal behind it is to change firefox UI to be as minimalistic as possible whenever it's not being actively used without sacrificing information, so for example all toolbar buttons will disappear when not being hovered on and so on.
Replace your userchrome.css file with the one available in this repository. For more information, please visit: https://www.howtogeek.com/334716/how-to-customize-firefoxs-user-interface-with-userchrome.css/
Tested on latest firefox nightly (62 alpha as of writing this)
The demonstration image feature the "Vivaldifox" theme that basically uses an algorithom to skin the UI with the most dominant color of the currently visited website