Unzip it, take the usercontent.css and the other folders and place it in /home/user/.mozilla/microb/chrome

If you don't know how to get there, well search for it on this forum. Just look up usercontent.css or chrome directory.

Edit: Thanks brontide, I'm going have to sift through this and add the css code I want.
Edit: By URL prefix to google.maps do you mean.. the "www" prefix? Because I tried that and it still doesn't work (just shows a gray box where Google Maps should be). Didn't notice any differences from that CSS code and the one that's been presented for full screen Google Maps though (which I had working).