--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -1947,17 +1947,16 @@ AC_SUBST(AS)
 AC_SUBST(ASFLAGS)
 AC_SUBST(AS_DASH_C_FLAG)
 AC_SUBST(RC)
 AC_SUBST(RCFLAGS)
 AC_SUBST(WINDRES)
 AC_SUBST(IMPLIB)
 AC_SUBST(FILTER)
 AC_SUBST(BIN_FLAGS)
-AC_SUBST(MOZ_DEBUG)
 AC_SUBST(MOZ_DEBUG_LDFLAGS)
 AC_SUBST(WARNINGS_AS_ERRORS)
 AC_SUBST(LIBICONV)
 
 AC_SUBST(ENABLE_STRIP)
 AC_SUBST(PKG_SKIP_STRIP)
 AC_SUBST(INCREMENTAL_LINKER)
 

--- a/moz.configure
+++ b/moz.configure
@@ -96,18 +96,24 @@ set_define('GTEST_USE_OWN_TR1_TUPLE',
 set_define('GTEST_HAS_CLONE',
            linux_gtest_defines.has_clone)
 
 js_option('--enable-debug',
           nargs='?',
           help='Enable building with developer debug info '
                '(using the given compiler flags).')
 
-add_old_configure_assignment('MOZ_DEBUG',
-                             depends('--enable-debug')(lambda v: bool(v)))
+@depends('--enable-debug')
+def moz_debug(debug):
+    if debug:
+        return bool(debug)
+
+set_config('MOZ_DEBUG', moz_debug)
+set_define('MOZ_DEBUG', moz_debug)
+add_old_configure_assignment('MOZ_DEBUG', moz_debug)
 
 js_option('--enable-rust-debug',
           help='Build Rust code with debug assertions turned on.')
 
 @depends('--enable-rust-debug', '--enable-debug')
 def debug_rust(value, debug):
     if value.origin == 'default':
         return bool(debug) or None

--- a/old-configure.in
+++ b/old-configure.in
@@ -4480,17 +4480,16 @@ AC_SUBST(AS_DASH_C_FLAG)
 AC_SUBST(RC)
 AC_SUBST(RCFLAGS)
 AC_SUBST(WINDRES)
 AC_SUBST(IMPLIB)
 AC_SUBST(FILTER)
 AC_SUBST(BIN_FLAGS)
 AC_SUBST(MOZ_AUTH_EXTENSION)
 AC_SUBST(MOZ_PREF_EXTENSIONS)
-AC_SUBST(MOZ_DEBUG)
 AC_SUBST(MOZ_DEBUG_LDFLAGS)
 AC_SUBST(WARNINGS_AS_ERRORS)
 AC_SUBST_SET(MOZ_EXTENSIONS)
 AC_SUBST(LIBICONV)
 AC_SUBST(MOZ_TOOLKIT_SEARCH)
 AC_SUBST(MOZ_FEEDS)
 
 AC_SUBST(MOZ_UNIVERSALCHARDET)

--- a/python/mozbuild/mozbuild/action/generate_symbols_file.py
+++ b/python/mozbuild/mozbuild/action/generate_symbols_file.py
@@ -24,17 +24,17 @@ def generate_symbols_file(output, *args)
     pp = Preprocessor()
     pp.context.update(buildconfig.defines['ALLDEFINES'])
     if args.D:
         pp.context.update(args.D)
     for undefine in args.U:
         if undefine in pp.context:
             del pp.context[undefine]
     # Hack until MOZ_DEBUG_FLAGS are simply part of buildconfig.defines
-    if buildconfig.substs['MOZ_DEBUG']:
+    if buildconfig.substs.get('MOZ_DEBUG'):
         pp.context['DEBUG'] = '1'
     # Ensure @DATA@ works as expected (see the Windows section further below)
     if buildconfig.substs['OS_TARGET'] == 'WINNT':
         pp.context['DATA'] = 'DATA'
     else:
         pp.context['DATA'] = ''
     pp.out = StringIO()
     pp.do_filter('substitution')

--- a/python/mozbuild/mozbuild/frontend/gyp_reader.py
+++ b/python/mozbuild/mozbuild/frontend/gyp_reader.py
@@ -143,17 +143,17 @@ def process_gyp_result(gyp_result, gyp_d
         # The list of included files returned by gyp are relative to build_file
         for f in data[build_file]['included_files']:
             context.add_source(mozpath.abspath(mozpath.join(
                 mozpath.dirname(build_file), f)))
 
         spec = targets[target]
 
         # Derive which gyp configuration to use based on MOZ_DEBUG.
-        c = 'Debug' if config.substs['MOZ_DEBUG'] else 'Release'
+        c = 'Debug' if config.substs.get('MOZ_DEBUG') else 'Release'
         if c not in spec['configurations']:
             raise RuntimeError('Missing %s gyp configuration for target %s '
                                'in %s' % (c, target_name, build_file))
         target_conf = spec['configurations'][c]
 
         if 'actions' in spec:
           handle_actions(spec['actions'], context, action_overrides)
         if 'copies' in spec: