I spend most of my day in front of Google Chrome, Mozilla Thunderbird, and Gnome Terminal on my RHEL 7 desktop running Gnome Shell. Chrome’s ability to hide the system titlebar creates a pretty slick and efficient desktop, so I’ve been craving the same for Thunderbird. Luck (and a little bit of Google-fu) just brought me to the Pixel Saver Gnome Shell extension. Works like a champ, thank you @deadalnix !