Firefox comes included with a lot of different color themes for customization. If you want to be able to add your own personal touches to your browser's interface, Stratiform can give you that flexibility.
You can change Firefox directly from the browser's interface with Stratiform. Just click the little icon to the right of the bookmarks. A tab will open that gives you a variety of customization options that can change your toolbar, the tabs, and other add-ons and buttons. You can even change the appearance of text fields.
Stratiform mostly affects the appearance of colors, as well as the size of certain aspects in Firefox (such as the tabs), but it allows you to specify exactly how you want areas of the browser to look, which is nice.
Unfortunately, Stratiform isn't compatible with all other add-ons available in Firefox, so it might run into issues, depending on what else you already have installed.
Stratiform is a great add-on that gives you a wide level of customizability when it comes to the appearance of Firefox's UI.