Favicons could show some useful information, not just a static image. If you’d like to see the number of unread messages next to Gmail’s red envelope, enable “Unread message icon” in Gmail Labs. The feature only works in Chrome and Firefox, but it’s not very reliable.
“When you’re visiting sites other than Gmail, it’s easy to find out how many unread messages are in your inbox by glancing at the title of your Gmail tab or window. However, if you have a ton of tabs open, or if you use Chrome’s Pin Tab feature that hides everything except the tab’s icon, it can be tricky to figure out without switching tabs,” mentions Gmail’s blog.
If you’d like even more features, Lifehacker suggests to use Minimalist Gmail, a Chrome extension that not only adds an unread counter to Gmail’s favicons, but it also lets you customize the navigation bar and remove extraneous features like the “invite a friend” box or the cluttered footer. Minimalist Gmail’s counter only shows the number of unread messages from the inbox.
{ Thanks, Greg. }