Currently, mail windows get the same "N" icon that browser windows get. This makes it hard to spot the mail window when you have many browser windows open and do an Alt-Tab in Windows. The icons are also the same in the taskbar. One more thing: The taskbar button for mail/news just says "Mail" whereas in 4.x it said the name of the current message title. (Maybe this should be a separate bug.)
I think this is a duplicate... will look for the other(s) bug.
Verified a duplicate of bug 47779 - Icon should be different from that of browser.