Firefox is meant to be customized, and it hands you the tools to do so. There are thousands of Firefox add-ons available, in addition to the host of options to customize your toolbars, search engines, and the very functionality of your browser.
That said, Firefox can still look pretty plain at times. Thankfully, Firefox also has an entire library of themes, often called Firefox personas, that you can add to overhaul the whole look of your browser.
How to Change Your Firefox Theme
Open Firefox, and select the three-line menu icon in the upper right side of the window.
Select Customize in the menu. It has a paintbrush icon next to it.
The customization tab will open. Here, you can customize a lot of the look and functionality of your browser. Drag any of the icons in the body of the window to your toolbar to make them a permanent fixture.
At the bottom of the window, you'll see a couple of checkboxes that let you change the look of your Firefox window a bit. There are also three drop-down menus. Select Themes.
The menu lists the three default themes at the top. Below them, Firefox suggests a few popular themes, but select Get More Themes at the bottom of the window instead.
Firefox will open yet another tab containing its theme library. Across the middle of the page are links to categories to help you better locate something you'll like. The bottom portion of the page is broken into rows of featured, top rated, and trending themes. Select the category you'd like to explore.
You'll find the same three row structure on your category page. Select See more in the top right of each row to get the full list for each.
The next page contains a more complete listing of themes and will give a small preview of your theme in a basic Firefox layout. Select a theme you might want to install.
On the theme's page, you'll see more information about it, including a more detailed breakdown of the theme's rating and additional themes by the same designer. To install your theme, select Install Theme under the theme preview.
Firefox will automatically install and apply your theme.
You can change your themes at any time, and have several installed at once. Return to the Firefox customization tab and select Themes > Manage.
Here, you'll see a listing of all your installed themes. Simply select Enable next to the one you want to use. If you ever want to remove a theme, select Remove.