Comment 70 for bug 481541

browser.tabs.loadDivertedInBackground can be set to true in about:config to achieve this. This is a pref that doesn't need UI, either, so I'm not sure what the point is, exactly. If you call focus() on window.content, that will focus the window (as it rightly should) and if you create and select a new tab, that's what we do. Short of adding even more complexity to tabbrowser, I don't see a way to open in foreground while delaying focus without a core-level fix that would allow us to call focusIfAndWhenWindowIsFocused() or something instead.