On 17.04, the top bar showed a mail icon. Clicking on it allowed me to open Thunderbird main, open a new message only or open the Thunderbird address book. I am sure there is a way but have been unable to find that in the questions or in the (Gnome ) Tweaks. How can I make that happen?
indicator-messages. Not sure about ordinary 17.10. Maybe Mailnag may trigger it viamailnag-messagingmenu-pluginorgnome-shell-mailnag.