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 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 more information, see wikipedia's article on the topic.