In the Customize Firefox screen that opens up, you can click on the Themes button near the lower edge of the window to choose one of the themes recommended by Firefox.

Then click on Customize in the menu that opens up.

Click on the settings icon (it looks like three horizontal lines, some people also call it burger icon) near the right corner of Firefox window.

You can follow these simple steps to install new themes, preview them in Firefox or apply the new themes: Now you can preview and apply the themes through the customization interface of Firefox. But with the latest version (version 34), Firefox has now made it very easy to choose, apply and install new themes in Firefox. Until now if you wanted to change the theme for Firefox, then you had to go through the menus and options to choose and apply a theme. One of the main reasons why Mozilla Firefox has become so popular is the flexibility it provides to the user to customize and add new features through the thousands of themes and extensions available online.

How to customize mozilla firefox themes