Badge Checker for Gmail™ is a lite add-on that help you quickly get badge notification for all unread emails in your Gmail inbox.
This addon supports multi-account, in other words, if you are logged in to more than one Gmail account, all unread emails in all your accounts are summed up and shown in the badge icon. Moreover, badge icon title shows the detailed description about badge number (badge number break down).
There is also an options page where you can adjust few settings including total number of Gmail accounts that you have; time interval to check Gmail account and your default Gmail account to open when clicking on the toolbar icon.
Note #1: this addon only shows unread emails for the "inbox" folder. Currently it does not support labels.
Note #2: in order to report bugs, please fill the bug report form in the addon's homepage (http://mybrowseraddon.com/badge-checker-for-gmail.html).