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diff --git a/lib/pref_observer/pref_observer.js b/lib/pref_observer/pref_observer.js
new file mode 100644
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+++ b/lib/pref_observer/pref_observer.js
@@ -0,0 +1,71 @@
+/**
+ * GNU LibreJS - A browser add-on to block nonfree nontrivial JavaScript.
+ * *
+ * Copyright (C) 2014 Loic J. Duros
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see  <http://www.gnu.org/licenses/>.
+ *
+ */
+
+var {Cc, Ci} = require("chrome");
+const httpObserver = require("http_observer/http_request_observer");
+
+// https://developer.mozilla.org/en-US/Add-ons/Code_snippets/Preferences#Using_preference_observers
+var prefObserver = {
+    register: function() {
+        // First we'll need the preference services to look for preferences.
+        var prefService = Cc["@mozilla.org/preferences-service;1"]
+            .getService(Ci.nsIPrefService);
+
+        // For this.branch we ask for the preferences for
+        // extensions.myextension. and children
+        this.branch = prefService.getBranch("javascript.");
+
+        // Finally add the observer.
+        this.branch.addObserver("", this, false);
+    },
+
+    unregister: function() {
+        this.branch.removeObserver("", this);
+    },
+
+    observe: function(aSubject, aTopic, aData) {
+        // aSubject is the nsIPrefBranch we're observing (after appropriate QI)
+        // aData is the name of the pref that's been changed (relative to
+        // aSubject)
+        switch (aData) {
+            case "enabled":
+                var prefs = require('sdk/preferences/service');
+                var isJavaScriptEnabled = prefs.get('javascript.enabled');
+                if (!isJavaScriptEnabled) {
+                    console.debug('JS disabled in observer');
+                    // remove all http notifications
+                    httpObserver.removeHttpObserver();
+
+                    // TODO: the narcissus worker could also be stopped at this
+                    // point, but I'm not doing that right now because I don't
+                    // know how to re-enable it.
+                    // narcissusWorker.stopWorker();
+                } else {
+                    console.debug('JS enabled in observer');
+                    httpObserver.startHttpObserver();
+                }
+                break;
+        }
+    }
+};
+
+exports.register = function() {
+    prefObserver.register();
+};