Bug 814530 - Disable @supports on Release and Beta. r=dbaron

modules/libpref/src/init/all.js

@@ -1639,7 +1639,11 @@ pref("layout.css.dpi", -1);
 pref("layout.css.devPixelsPerPx", "-1.0");
 
 // Is support for the the @supports rule enabled?
+#ifdef RELEASE_BUILD
+pref("layout.css.supports-rule.enabled", false);
+#else
 pref("layout.css.supports-rule.enabled", true);
+#endif
 
 // Is support for CSS Flexbox enabled?
 pref("layout.css.flexbox.enabled", true);