There are some regressions causing delays for the release of Thunderbird 38.1.0 (which was planned for June 30th). On 06/22/15 in TB-planning Kent posted:
The current plan for Thunderbird 38.1.0 is to land patches on Monday, June 29 and try to release at the end of that week. This will have a number of fixes to regressions that were uncovered in the initial release.
Two weeks later the first Release Candidate (RC) for 38.1.0 has been pushed out for the testers. If everything goes well with this RC then we could see the release of Thunderbird 38.1.0 by the end of the week.