diff --git a/python/mozbuild/mozbuild/backend/recursivemake.py b/python/mozbuild/mozbuild/backend/recursivemake.py
index 42b8ad541d8..c5ebae81be7 100644
--- a/python/mozbuild/mozbuild/backend/recursivemake.py
+++ b/python/mozbuild/mozbuild/backend/recursivemake.py
@@ -409,11 +409,9 @@ class RecursiveMakeBackend(CommonBackend):
         self._traversal = RecursiveMakeTraversal()
         self._compile_graph = defaultdict(set)
 
-        self._may_skip = {
+        self._no_skip = {
             'export': set(),
             'libs': set(),
-        }
-        self._no_skip = {
             'misc': set(),
             'tools': set(),
         }
@@ -626,18 +624,12 @@ class RecursiveMakeBackend(CommonBackend):
         convenience variables, and the other dependency definitions for a
         hopefully proper directory traversal.
         """
-        for tier, skip in self._may_skip.items():
-            self.log(logging.DEBUG, 'fill_root_mk', {
-                'number': len(skip), 'tier': tier
-                }, 'Ignoring {number} directories during {tier}')
         for tier, no_skip in self._no_skip.items():
             self.log(logging.DEBUG, 'fill_root_mk', {
                 'number': len(no_skip), 'tier': tier
                 }, 'Using {number} directories during {tier}')
 
         def should_skip(tier, dir):
-            if tier in self._may_skip:
-                return dir in self._may_skip[tier]
             if tier in self._no_skip:
                 return dir not in self._no_skip[tier]
             return False
@@ -792,10 +784,13 @@ class RecursiveMakeBackend(CommonBackend):
                         {'path': makefile}, 'Substituting makefile: {path}')
                     self._makefile_in_count += 1
 
-                    for tier, skiplist in self._may_skip.items():
-                        # topobjdir is an exception, it's still skipped.
-                        if bf.relobjdir and bf.relobjdir in skiplist:
-                            skiplist.remove(bf.relobjdir)
+                    # In the export and libs tiers, we don't skip directories
+                    # containing a Makefile.in.
+                    # topobjdir is handled separatedly, don't do anything for
+                    # it.
+                    if bf.relobjdir:
+                        for tier in ('export', 'libs',):
+                            self._no_skip[tier].add(bf.relobjdir)
                 else:
                     self.log(logging.DEBUG, 'stub_makefile',
                         {'path': makefile}, 'Creating stub Makefile: {path}')
@@ -922,9 +917,6 @@ class RecursiveMakeBackend(CommonBackend):
 
         affected_tiers = set(obj.affected_tiers)
 
-        for tier in set(self._may_skip.keys()) - affected_tiers:
-            self._may_skip[tier].add(backend_file.relobjdir)
-
         for tier in set(self._no_skip.keys()) & affected_tiers:
             self._no_skip[tier].add(backend_file.relobjdir)