--- a/client.mk
+++ b/client.mk
@@ -47,18 +47,16 @@ endef
 # As $(shell) doesn't preserve newlines, use sed to replace them with an
 # unlikely sequence (||), which is then replaced back to newlines by make
 # before evaluation. $(shell) replacing newlines with spaces, || is always
 # followed by a space (since sed doesn't remove newlines), except on the
 # last line, so replace both '|| ' and '||'.
 MOZCONFIG_CONTENT := $(subst ||,$(CR),$(subst || ,$(CR),$(shell cat $(OBJDIR)/.mozconfig-client-mk | sed 's/$$/||/')))
 include $(OBJDIR)/.mozconfig-client-mk
 
-export FOUND_MOZCONFIG
-
 # As '||' was used as a newline separator, it means it's not occurring in
 # lines themselves. It can thus safely be used to replaces normal spaces,
 # to then replace newlines with normal spaces. This allows to get a list
 # of mozconfig output lines.
 MOZCONFIG_OUT_LINES := $(subst $(CR), ,$(subst $(NULL) $(NULL),||,$(MOZCONFIG_CONTENT)))
 # Filter-out comments from those lines.
 START_COMMENT = \#
 MOZCONFIG_OUT_FILTERED := $(filter-out $(START_COMMENT)%,$(MOZCONFIG_OUT_LINES))

--- a/python/mozbuild/mozbuild/controller/building.py
+++ b/python/mozbuild/mozbuild/controller/building.py
@@ -1334,16 +1334,17 @@ class BuildDriver(MozbuildObject):
             if mozconfig['make_flags']:
                 fh.write(b'MOZ_MAKE_FLAGS=%s\n' % b' '.join(mozconfig['make_flags']))
             objdir = mozpath.normsep(self.topobjdir)
             fh.write(b'MOZ_OBJDIR=%s\n' % objdir)
             fh.write(b'OBJDIR=%s\n' % objdir)
             if mozconfig['path']:
                 fh.write(b'FOUND_MOZCONFIG=%s\n' %
                          mozpath.normsep(mozconfig['path']))
+                fh.write(b'export FOUND_MOZCONFIG\n')
 
         append_env['OBJDIR'] = mozpath.normsep(self.topobjdir)
 
         return self._run_make(srcdir=True,
                               filename='client.mk',
                               allow_parallel=False,
                               ensure_exit_code=False,
                               print_directory=False,