Comment 4 for bug 1491249

George99 on the Linux Mint Forums noticed something possibly relevant in the changelog for the repository version of 38.2.0:
https://launchpad.net/ubuntu/+source/thunderbird/1:38.2.0+build1-0ubuntu0.14.04.1

    * Drop debian/patches/revert-removal-of-native-notifications.patch - fixed upstream