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 at Component.utils page. 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.

See also the effects that XPCNativeWrappers have on the Sandbox's relationship to the content page.