Firefox puts the title of the active tab at the top of the window. It's a very useful, helpful UI element. Look and see how the window title shifts as I flip through the tabs in Firefox:
Orion does not provide a title at the top of the window. There's nothing above the URL bar. Look and see how there's still nothing above the URL bar, no matter how wistfully I flip through the tabs in Orion:
Having the title at the top of the window makes it much easier to find and click the desired one of many windows open at the same time. It also makes it much easier to grab and drag the window. I cannot imagine a drawback, but perhaps there's one I'm not thinking of; if that's the case, how about making this feature a togglable option?