diff --git a/extensions/firefox/content/PdfjsChromeUtils.jsm b/extensions/firefox/content/PdfjsChromeUtils.jsm
index 7adb549fd8a2e..b5f2a62f25028 100644
--- a/extensions/firefox/content/PdfjsChromeUtils.jsm
+++ b/extensions/firefox/content/PdfjsChromeUtils.jsm
@@ -53,7 +53,7 @@ var PdfjsChromeUtils = {
    */
 
   init: function () {
-    this._browsers = new Set();
+    this._browsers = new WeakSet();
     if (!this._ppmm) {
       // global parent process message manager (PPMM)
       this._ppmm = Cc['@mozilla.org/parentprocessmessagemanager;1'].