Starting with Monday's nightly we've discovered that we're missing mac
symbols for the XUL library. Because this makes crash statistics useless,
this means that we can't bisect or detect most crash issues on mac. For
that reason, I've asked sheriffs to close the tree.
This is being tracked in bug 1380381, and is a recurrence of an older issue
from bug 1371017 and bug 1301751.
Currently these sorts of symbol errors don't fail the build and so we don't
notice them very quickly, but that is being tracked in bug 1304042 and I
hope we can land that soon to avoid a recurrence of this issue.