--- a/toolkit/modules/AppConstants.jsm
+++ b/toolkit/modules/AppConstants.jsm
@@ -255,16 +255,23 @@ this.AppConstants = Object.freeze({
 
   MOZ_REQUIRE_SIGNING:
 #ifdef MOZ_REQUIRE_SIGNING
   true,
 #else
   false,
 #endif
 
+  MOZ_ALLOW_LEGACY_EXTENSIONS:
+#ifdef MOZ_ALLOW_LEGACY_EXTENSIONS
+  true,
+#else
+  false,
+#endif
+
   INSTALL_COMPACT_THEMES:
 #ifdef INSTALL_COMPACT_THEMES
   true,
 #else
   false,
 #endif
 
   MENUBAR_CAN_AUTOHIDE:

--- a/toolkit/moz.configure
+++ b/toolkit/moz.configure
@@ -533,16 +533,20 @@ project_flag('MOZ_SERVICES_HEALTHREPORT'
 
 project_flag('MOZ_SERVICES_SYNC',
              help='Build Sync Services if required')
 
 project_flag('MOZ_ANDROID_HISTORY',
              help='Enable Android History instead of Places',
              set_as_define=True)
 
+project_flag('MOZ_ALLOW_LEGACY_EXTENSIONS',
+             help='Allow legacy browser extensions',
+             default=True, set_as_define=True)
+
 option(env='MOZ_PHOTON_ANIMATIONS',
        help='Enable Photon UI animations',
        default=is_nightly)
 
 @depends('MOZ_PHOTON_ANIMATIONS')
 def photon_animations(value):
     if value:
         return bool(value)