Copy the link for the theme of your choice to your clipboard, and then go to the Firefox theme install page.
The last step is to paste the link from your clipboard into the "Remote URL" field to install the theme into Firefox. Note: you must do this using Firefox for the install to work! You must also select the theme in the Tools -> Themes dialog to activate it, and then restart to have it take effect.
[robg adds: These themes do look quite nice, however, they are designed for Windows. That means that you'll get a functional but not right looking scrollbar on OS X. I'm presently using a set of themes built by 'arronax07' specifically for OS X; these have nicer looking widgets than the stock Firefox, and feature full scrollbar support...]

