The DOM beforeload event is a WebKit concept, it is not specific to Safari extensions. It does have a URL property.
For some more information about the beforeload event - you can see:
http://opensource.apple.com/source/WebCore/WebCore-1298/dom/BeforeLoadEvent.idl
If you are using this in an extension content script, one way to get some more information about be to have some code like:
document.addEventListener("beforeload", beforeLoadHandler, false);
function beforeLoadHandler(event)
{
console.log(event);
}
And then use the Web Inspector to view information about the event (you can see its URL, you can look at the event target, etc).
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