Sandbox

For security reasons, modern versions of Greasemonkey execute each script in a "sandbox", which isolates trusted user script code from potentially malicious web page code.

Documentation for Component.utils.Sandbox^[1] may eventually show up on the Component.utils page, but it not there now. There is documentation for Components.utils.evalInSandbox which gives an example of Sandbox usage.

For information on sandboxes in general, see Wikipedia's article on the topic.