While Gmail Labs already featured an "unread message count in title-bar" hack, the same was not very useful while using tab-pinning in Firefox or Chrome web-browsers. Thanks to a new Gmail Labs feature we can now have the unread mail count as an favicon clearly visible when using tab-pinning feature in Firefox and Google Chrome.

Gmail unread message count favicon with 100+ such emails

To activate simple visit the "green beaker" icon located at the top-right side of your Gmail interface and Enable "Unread message icon" from there.