Credits to Harvey Cabaguio for setting the theme up, Surendrajat, DRHAX34 and sempasha for the install script and the elementary team for the UI design and icons.

I recommend installing Firefox in the terminal, typing: sudo apt install firefox, because Firefox in the Appcenter is a flatpak version which doesn't work well with the userChrome of this theme.

Firefox does not support userChrome.css by default. Here are the steps to make it work:

1. Load about:config in the Firefox address bar.
2. Confirm that you will be careful.
3. Search for toolkit.legacyUserProfileCustomizations.stylesheets using the search at the top.
4. Toggle the preference by double clicking. True means Firefox supports the CSS files, False that it ignores them.

Follow this step to install the userChrome.css:

You can set 8 different window buttons layouts with Pantheon Tweaks. Now it's possible to install this theme to match these window button layouts.

Use this one line install script which let you select the window buttons layout you like:

curl -s -o- https://raw.githubusercontent.com/Zonnev/elementaryos-firefox-theme/elementaryos-odin-firefox-theme/install.sh | bash

Just copy the line to your terminal and press enter. Then it lets you choose which window buttons layout you need (1-8) and press enter again. If you don't want to install elementary Tweaks, then you select the default layout (option 1 in the installer).

1. Disable Title bar in the customization panel in Firefox.
2. In the customization panel in Firefox you can move the new tab button to the left if you wish and use the dark mode option as well, in combination with Pantheon Tweaks.