The script works great in Firefox, however not in Safari when using GreaseKit.

This was fixed by wrapping it in an anonymous function.

(function(Twitter){
// wrap everything in an anonymous function because this script doesn't run
// in a wrapper when used in GreaseKit or other contexts other than Firefox

(function() {

var userTwitterURL;
var follower = false; // assume on friends page
var followTableXPath;
var userRowElem;
var userTdElem2;
var userLinkElem;

// determine if on friends or followers page
var followersPattern = /followers/;
if (followersPattern.test(document.location)) {
follower = true;
}
}

I figure this out from the script source below:

http://userscripts.org/scripts/show/24398

It now works in Safari.

Ooops... I was mistaken. It still does not work in Safari with GreaseKit. Hmmmm?