Most of the mac gitian descriptors are using MacOSX10.7.sdk.tar.gz. I don't know the source of that originally.
In working on the mac build for Snowflake, I had to apply a few workarounds for features that are not present in the 10.7 SDK, which would be unnecessary if using the 10.9 SDK. https://bugs.torproject.org/19001#comment:32
The blog announcement for Tor Browser 7.0 notes that it drops support for macOS 10.7 and now requires 10.9. https://bugs.torproject.org/18530
Does that mean that the Firefox build uses the 10.9 SDK, while the other descriptors still use the 10.7 SDK? (It wasn't clear to me reading the source code. mac/gitian-firefox.yml unpacks MacOSX10.7.sdk.tar.gz but doesn't seem to use it.) What does it take to use a more recent SDK in other descriptors (especially mac/gitian-webrtc.yml)?