https://web.archive.org/web/20190223133900/https://addons.mozilla.org/en-US/firefox/complete-themes/
Like many programs, Firefox can be themed.
As popular as Firefox is, out of 397 themes I only found three I wanted to try. All three failed my review.
The one which came closest to being usable was ChromiFox Basic, but I disqualified it when they took the typeahead search within the URL field and ruined its readability. =(