An adaptation of the Hobbit theme by UltraPeter for Google Chrome for Firefox that I threw together. I recently switched to using Mozilla Firefox as my default browser from Google Chrome & I really liked the Hobbit theme I was using earlier, so I decided to recreate it. Feel free to add to your own Firefox setup!
Link to Original Theme: https://ultrapeter.com/chrome/detail.php?i=21
Link to Chrome Web Store: https://chromewebstore.google.com/detail/the-hobbit/miabjcilknnjnfeikobfhbfkhjcfhpbe
The chrome folder goes under your default profile folder after enabling the legacy stylesheets permission in about:config. This will allow you to change the home page background picture in Firefox to match the Hobbit theme.
The theme itself can be imported by the signed .xpi file you can find under releases. Note that due to the way Firefox works with themes, this only adjusts the menu & address bars' appearances. It also doesn't look as good if you enable the toolbar using the Alt shortcut, so your milage may vary depending on how you have your layout arranged.