XZS  filed a request to merge thunderbird-extension-gnome-integration  into mozilla-extension-gnotifier-git :
The gnome-integration extension is no longer under active development and not compatible with the current Thunderbird version. Gnotifier provides the same functionality for Thunderbird and Firefox alike and is up to date, so users are advised to switch to it. Merging the packages would reflect this.
 https://aur4.archlinux.org/account/XZS/  https://aur4.archlinux.org/pkgbase/thunderbird-extension-gnome-integration/  https://aur4.archlinux.org/pkgbase/mozilla-extension-gnotifier-git/