--- a/client.mk
+++ b/client.mk
@@ -32,37 +32,18 @@ ifeq "$(CWD)" "/"
 CWD   := /.
 endif
 
 PYTHON ?= $(shell which python2.7 > /dev/null 2>&1 && echo python2.7 || echo python)
 
 ####################################
 # Load mozconfig Options
 
-# See build pages, http://www.mozilla.org/build/ for how to set up mozconfig.
-
-define CR
-
-
-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
 
-# 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)))
-
 ifdef MOZ_PARALLEL_BUILD
   MOZ_MAKE_FLAGS := $(filter-out -j%,$(MOZ_MAKE_FLAGS))
   MOZ_MAKE_FLAGS += -j$(MOZ_PARALLEL_BUILD)
 endif
 
 # Automatically add -jN to make flags if not defined. N defaults to number of cores.
 ifeq (,$(findstring -j,$(MOZ_MAKE_FLAGS)))
   cores=$(shell $(PYTHON) -c 'import multiprocessing; print(multiprocessing.cpu_count())')
@@ -87,26 +68,16 @@ CONFIGURES += $(TOPSRCDIR)/js/src/config
 # The default rule is build
 build::
 
 ifndef MACH
 $(error client.mk must be used via `mach`. Try running \
 `./mach $(firstword $(MAKECMDGOALS) $(.DEFAULT_GOAL))`)
 endif
 
-# For now, only output "export" lines and lines containing UPLOAD_EXTRA_FILES.
-MOZCONFIG_MK_LINES := $(filter export||% UPLOAD_EXTRA_FILES% %UPLOAD_EXTRA_FILES%,$(MOZCONFIG_OUT_LINES))
-$(OBJDIR)/.mozconfig.mk: $(TOPSRCDIR)/client.mk $(FOUND_MOZCONFIG)
-	$(if $(MOZCONFIG_MK_LINES),( $(foreach line,$(MOZCONFIG_MK_LINES), echo '$(subst ||, ,$(line))';) )) > $@
-
-# Include that makefile so that it is created. This should not actually change
-# the environment since MOZCONFIG_CONTENT, which MOZCONFIG_OUT_LINES derives
-# from, has already been eval'ed.
-include $(OBJDIR)/.mozconfig.mk
-
 # In automation, manage an sccache daemon. The starting of the server
 # needs to be in a make file so sccache inherits the jobserver.
 ifdef MOZBUILD_MANAGE_SCCACHE_DAEMON
 build::
 	# Terminate any sccache server that might still be around.
 	-$(MOZBUILD_MANAGE_SCCACHE_DAEMON) --stop-server > /dev/null 2>&1
 	# Start a new server, ensuring it gets the jobserver file descriptors
 	# from make (but don't use the + prefix when make -n is used, so that

--- a/python/mozbuild/mozbuild/controller/building.py
+++ b/python/mozbuild/mozbuild/controller/building.py
@@ -1348,21 +1348,34 @@ class BuildDriver(MozbuildObject):
         mozconfig_make_lines.append(b'MOZ_OBJDIR=%s' % objdir)
         mozconfig_make_lines.append(b'OBJDIR=%s' % objdir)
 
         if mozconfig['path']:
             mozconfig_make_lines.append(b'FOUND_MOZCONFIG=%s' %
                                         mozpath.normsep(mozconfig['path']))
             mozconfig_make_lines.append(b'export FOUND_MOZCONFIG')
 
+        # The .mozconfig.mk file only contains exported variables and lines with
+        # UPLOAD_EXTRA_FILES.
+        mozconfig_filtered_lines = [
+            line for line in mozconfig_make_lines
+            # Bug 1418122 investigate why UPLOAD_EXTRA_FILES is special and
+            # remove it.
+            if line.startswith(b'export ') or b'UPLOAD_EXTRA_FILES' in line
+        ]
+
         mozconfig_client_mk = os.path.join(self.topobjdir,
                                            '.mozconfig-client-mk')
         with FileAvoidWrite(mozconfig_client_mk) as fh:
             fh.write(b'\n'.join(mozconfig_make_lines))
 
+        mozconfig_mk = os.path.join(self.topobjdir, '.mozconfig.mk')
+        with FileAvoidWrite(mozconfig_mk) as fh:
+            fh.write(b'\n'.join(mozconfig_filtered_lines))
+
         if mozconfig_make_lines:
             self.log(logging.WARNING, 'mozconfig_content', {
                 'path': mozconfig['path'],
                 'content': '\n    '.join(mozconfig_make_lines),
             }, 'Adding make options from {path}\n    {content}')
 
         append_env['OBJDIR'] = mozpath.normsep(self.topobjdir)