If you want to do the update manually, when you feel like it or when you know the all important plugins you use are compatible with the new version, you may follow the instructions to disable automatic update of Thunderbird and Firefox. I found that previous version of Thunderbird and Firefox are not always easy to find on the Mozilla Website.
Session and Startup Thunderbird is not set to start.
Mozilla has decided to speed up the release cycle of Thunderbird and Firefox.
This is all good except that it often breaks Add-ons.
What you will install or upgrade to is the 38.5.1 one.