Tab Counter от Perry Naseck

Add a badge to the toolbar that shows the number of currently open tabs in a window. Click it to see the total number of open tabs in all windows. Badge color is configurable. The badge text color is configurable in Firefox v63 and above. Set the addon to use the clear icon to see only the number in the toolbar. Badge can be configured to show the count of the number of tabs in the current window, the number of tabs in all windows, the total number of windows, or neither (click the icon to see a popup).

This Addon supports e10s. This Addon is a WebExtension and will continue to work in Firefox 57.