diff --git a/js/colorbox.js b/js/colorbox.js
index ace202e..e0bd810 100644
--- a/js/colorbox.js
+++ b/js/colorbox.js
@@ -9,7 +9,9 @@ Drupal.behaviors.initColorbox = {
     if (settings.colorbox.mobiledetect && window.matchMedia) {
       // Disable Colorbox for small screens.
       var mq = window.matchMedia("(max-device-width: " + settings.colorbox.mobiledevicewidth + ")");
-      if (mq.matches) {
+      // In FireFox, window.matchMedia exists but returns null when used in a
+      // hidden iframe.
+      if (mq == null || mq.matches) {
         return;
       }
     }