You can download Mozilla Firefox free from Mozilla website. Firefox allows installing themes and add-ons. Install a new theme on Mozilla Firefox to change the appearance of toolbars and icons. So, follow few steps to know how to change Firefox theme.
Change Firefox Theme
1. Click the mouse right button up to the Mozilla Firefox shortcut icon to launch the Firefox browser. 
Open the Mozilla Theme’s website. Click on your web address bar, type this web address https://addons.mozilla.org/en-US/firefox/themes/ and press Enter from keyboard.
You will see Mozilla Firefox theme list. You can choose a theme from the category.
Click up to a theme. 
2. Click the Install Theme button with a + sign to change Firefox Theme. 
If you see any message then press OK button.
You will see your recently selected theme style on your Mozilla Firefox title banner. Using this way you can change your Firefox default theme and use a new theme to get a new look at Firefox.
