Welcome to Crystal XP ph1lwh0
I am assuming you have Firefox
1. Unzip/Unrar the downloaded folder (of course!
2. Open Firefox
3. Click on Tools
4. Then on Themes to open the themes window
Now also open the folder if the file is in the folder...
5. Just drag and drop the jar file into the theme window
6. Click on OK when Firefox asks you if you want to install the theme
7. Restart firefox
Now just open the themes window again and choose the theme and click on "Use Theme". Then just restart firefox again to see you theme in action.
Hope that helps
Enjoy your Stay