One of the new features in Firefox 4 is the App Tab which lets users persist a tab that they use continuously. Firefox shrinks the tab down to just the favicon and places it in a special area for these tabs which generally aren’t closed by the user. The feature is great, but one of the side effects is that Gmail App Tabs don’t show anymore the part of the
<title> that indicates unread messages.
Go install Unread Gmail Favicon from AMO and the favicon for that tab will indicate the number of unread messages when you have them like so: