Source (link to git-repo or to original if based on someone elses unmodified work): https://github.com/kurogetsusai/firefox-gnome-theme
This is a bunch of CSS code to make Firefox 57+ look closer to GNOME's default theme. It styles the UI and internal Firefox' pages like about: and view-source:. Both light and dark variants are supported.
The master branch is currently tracking Firefox 59. If you need support for older versions, they are archived under version-specific git tags. Eg. if you're using Firefox 57, switch to the 57 tag after cloning the repo:
git checkout 57