This won't be implemented. The reason is that the number on the Feedbro WebExtension icon tells the number of unread articles. The user might very well want to retain that information even after clicking the icon and reading a few articles. That's because the number would then remind the user that there are still some unread left.

If you want that number to go away, you can always use "Mark all as read" (first select the "All Items" main menu item and then click the green checkmark icon or press Shift+a) which will then mark all unread articles as read.