Browser Configuration/Firefox Breeze theme
Since Firefox Quantum, Firefox UI is based on an HTML+CSS UI. It's mean that we can modify the UI with web technology. This page list some project, that try to archive a better look for Firefox in a plasma environment.
This project adds the breeze icons to the Firefox interface. Support Breeze and Breeze dark.
Breeze Dark theme
There is a lot of project that add a Breeze dark color scheme Firefox.