Thunderbird updated for me and many others yesterday to Version 38.1.0 . Unfortunately there seem to be some problems with this new update. For me the reply stopped working or worse inserted the addressee in the earlier email ! As a quick fix I’ve reverted to an earlier version of Thunderbird which seems to be working as before. To do this  removed  the current version from the Control Panel (Windows) and then installed the earlier version. Earlier versions of Thunderbird can be found here:

You may also wish disable the updates see this link

Hope this is of help to my readers