Why Js Onunload Is Invoked When Popup Page Opens?

According to the documentation (I'm only using Firefox) : https://developer.mozilla.org/en/DOM/window.onunload The unload event is raised when the document is unloaded. But my

Solution 1:

You actually see the unload for the original about:blank document in the popup window first. (You can verify this by looking at the window's location.) You should then see another unload when the popup window closes.