Use Case: In a multiple window application a user can open a new window in the browser that will be a new page. If a long process is executed in the new (child) window and the user goes back to a parent window. Once the processing of the child window finishes, in IE only, the focus is moved to this new window.

If the user does no actions/interaction with the parent page then once the child window completes its processing the window is displayed to the user. If the user is interacting with the page ex: entering data/clicking, the focus is moved to the child window but the window is not displayed (due to the interactions with the parent page, the window is displayed but focus is set back to the parent page).

In Firefox, focus is not applied to the child window. The user is allowed to continue any processing on the parent window without interference from the child window.