browser.type

type
Type: one of the values below.
The type of browser, which can be used to set access of the document loaded inside the browser. If this is not set, the loaded document has the same access as the window containing the browser. More precisely: The document loaded into a chrome window is always of chrome type. Subdocuments of chrome documents are of chrome type, unless the container element (one of iframe, browser or editor) has one of the special type attribute values (the common ones are content, content-targetable and content-primary) indicating that the subdocument is of content type. This boundary has a number of special effects, such as making window.top == window (unless the browser is added to a chrome document), and preventing documents from inheriting the principal of the parent document. The type attribute on all frames in content documents is ignored; subdocuments of content documents are always content documents.
Warning: The type attribute must be set before the element is inserted into the document.
content
A browser for content. The content that is loaded inside the browser is not allowed to access the chrome above it.
content-primary
The primary browser for content. The content that is loaded inside the browser is not allowed to access the chrome above it. For instance, in a web browser, this would be the element that displays the web page. The window for the primary content can be retrieved more conveniently using window.content.
content-targetable
One browser among many for content. The content that is loaded inside the browser is not allowed to access the chrome above it. This is the preferred value for any browser element in an application, which will use multiple browsers of equal privileges, and is unselected at the moment.
chrome
(default behaviour): A browser, intended to be used for loading privileged content using a chrome:// URI. Don't use for content from web, as this may cause serious security problems!

See also

More information in the XUL tutorial and iframe