Tom, looking at the build logs, these releases use mozilla-beta as mozilla, which is gecko 58, not gecko 57. To get 57, you'll need to set up the automation against mozilla-release now. This might be triggering some of the problems we see here, not sure. It's sure to cause some confusion, I think.
Pike, thanks for noticing that. We are using the THUNDERBIRD_57_0b1_RELEASE tag which is branched off of 57.