The location hack is an older technique for crossing the barrier between the user script scope and the content window scope, when it exists. The previous version of this page may be read via history.
In Firefox version 39.0.3 Mozilla patched a known security vulnerability. As a side effect, this broke the location hack.
Thankfully in modern versions of Firefox and Greasemonkey, the location hack is no longer necessary. Read about how to execute code in the content scope, like previously possible with the location hack, at Content Script Injection.