As someone who generally has Gmail open in a tab all day long, its these little tweaks that make it a more enjoyable experience. That said, information at a glance is a good thing and the latest addition in the Gmail Labs does just that — allows you to see how many unread messages you have in your inbox by simply looking at the icon for the tab that you have Gmail open in.
This feature can be turned on by heading to the Labs section. From your Gmail account, click on the Settings link (upper right hand corner) and then go to Labs. From there scroll down until you see the “Unread message icon” section and click the checkbox for Enable. After that, scroll all the way down and click the Save Changes button. After that you will be able to see your unread message count in your browser tab which should keep your from clicking over just to see if you have something or not. As for support, this will work for those running version 6 or later of Chrome and version 2 or later of Firefox.
Via [Official Gmail Blog]