It is quite easy. Go here:
https://addons.mozilla.org/en-US/firefox/addon/greasemonkey/
Click add to firefox
You probably need to restart the browser once that is installed. Then go to:
http://userscripts.org/scripts/show/109145
Click on the green button in the top right corner.
Thats it .. go to the geocaching maps and the layers will now have an additional set of maps to choose from.
