--- a/python/mozbuild/mozbuild/artifacts.py
+++ b/python/mozbuild/mozbuild/artifacts.py
@@ -971,22 +971,25 @@ class Artifacts(object):
         """Returns recent ancestors of the working parent that are likely to
         to be known to Mozilla automation.
 
         If we're using git, retrieves hg revisions from git-cinnabar.
         """
         if self._git:
             return self._get_hg_revisions_from_git()
 
-        return subprocess.check_output([
+        # Mercurial updated the ordering of "last" in 4.3. We use revision
+        # numbers to order here to accommodate multiple versions of hg.
+        last_revs = subprocess.check_output([
             self._hg, 'log',
-            '--template', '{node}\n',
+            '--template', '{rev}:{node}\n',
             '-r', 'last(public() and ::., {num})'.format(
                 num=NUM_REVISIONS_TO_QUERY)
         ], cwd=self._topsrcdir).splitlines()
+        return [i.split(':')[-1] for i in sorted(last_revs, reverse=True)]
 
     def _find_pushheads(self):
         """Returns an iterator of recent pushhead revisions, starting with the
         working parent.
         """
 
         last_revs = self._get_recent_public_revisions()
         candidate_pushheads = self._pushheads_from_rev(last_revs[0].rstrip(),