Step 1:
Go to addons.mozilla.com

Step 2:
Click Themes on the left-hand toolbar.

Step 3:
Browse Themes by category or by all. When you find the one you want, click on the heading. You will be directed to the download page for that theme. On this page, it will tell you if the theme is mac or PC only and if it is compatible with select versions of the browser.

Step 4:
Click the button that says “Install Now”.

Step 5:
A new window will pop-up. Click “Install Now” again.

Step 6:
The next window is the Add-ons window. Click “Restart Firefox” to restart. It will end the session and then restart from the same URL. Any form data will be lost.

Step 7:
Now go to the tools menu and click Add-ons.

Step 8:
On the top of the next menu there is a tab for themes. Click that.

Step 9:
A short list of themes should appear. Click the one you want and then click “Use Theme”.

Step 10:
Close Firefox and restart and you theme will be in effect.