In some cases, Firefox will recognize the .jar file as a theme and prompt an “Install now” dialog.
Otherwise,
- Save the .jar file to your computer. If it is in a zip file, unzip (see How To Unzip A File).
- In Firefox, click Tools > Add-ons > Themes to open the Themes window
- Drag the .jar file into the left panel of the Themes window, which will prompt the “Install now” dialog.
Now that that the theme is installed it should be listed in the Themes window. Click “Use Theme” and restart Firefox for the theme to take effect.