Open Tab Count
This extension displays the number of open tabs for each window on the button and the total number of open tabs (in all windows) in a tip. This is based on the extension by Hilary Mason.
What is Open Tab Count?
Open Tab Count is a Firefox add-on that displays the number of open tabs for each window on the button and the total number of open tabs in a tip. It provides a convenient way to keep track of the number of tabs open in your Firefox browser.
Open Tab Count may not be safe to use and it requires some risky permissions. Exercise caution when installing this add-on. Review carefully before installing.
Open Tab Count has earned a fairly good reputation and likely can be trusted.