diff --git a/modules/libpref/src/init/all.js b/modules/libpref/src/init/all.js
index e7875615bd05..f6db35943ee4 100644
--- a/modules/libpref/src/init/all.js
+++ b/modules/libpref/src/init/all.js
@@ -1683,7 +1683,11 @@ pref("layout.css.supports-rule.enabled", true);
 #endif
 
 // Is support for CSS Flexbox enabled?
+#ifdef RELEASE_BUILD
+pref("layout.css.flexbox.enabled", false);
+#else
 pref("layout.css.flexbox.enabled", true);
+#endif
 
 // Are sets of prefixed properties supported?
 pref("layout.css.prefixes.border-image", true);