Thunderbird 38.4.0 has not yet been released. Reviewing the notes from the status meeting on November 3rd, the release was planned for around November 5th. This delay is not unusual and likely we will be seeing the Thunderbird 38.4.0 release during this coming week.
While going through the meeting notes did notice bug 1211160 which had to do with ‘some’ users who updated to Thunderbird 38.3 with Lightning 4.0.3 enabled lost use of the buttons in Thunderbird. Users reported that rolling back to Thunderbird 38.2.0 resolved the issue. Other uses reported success by disabling Lightning 4.0.3 prior to updating to the Thunderbird 38.3.0 release and then re-enabling lightning. There was an update to Lightning (220.127.116.11) back on October 8th which appeared to be work around for this issue (this did not resolve the issue as the bug is still showing as NEW & Need Info). If you have reverted back to Thunderbird 38.2.0 because of this issue, consider updating to Thunderbird 38.3.0 and then after the restart, update Lighting to the 18.104.22.168 version (does not work with Thunderbird 38.2.0 or older).
As I mentioned above ‘some’ user have had this issue. I did not experience this issue on either my Windows 7 or 8.1 machines when updating to Thunderbird 38.3.0 last month. However, I’ve been running Lightning on those machines when it was still a standalone add-on prior to it being integrated into Thunderbird. I’ve added this bug to the growing list of bugs I am following and will post an update when there is more info.