Cleaned up code.

.gitignore

@@ -0,0 +1,2 @@
+.jshintrc
+*.swp

background.js

@@ -1,8 +1,8 @@
-browser.idle.onStateChanged.addListener(function(state) {
+browser.idle.onStateChanged.addListener((state) => {
     if (state == 'idle') {
-        browser.storage.local.get('days', function(res) {
+        browser.storage.local.get('days').then((res) => {
             var days = res.days || 0;
-            if (days != 0) {
+            if (days !== 0) {
                 var end = new Date();
                 end.setHours(0);
                 end.setMinutes(0);

manifest.json

@@ -3,6 +3,11 @@
     "name": "History Cleaner",
     "version": "1.1",
     "description": "Deletes history older than a specified amount of days.",
+    "applications": {
+        "gecko": {
+            "id": "{a138007c-5ff6-4d10-83d9-0afaf0efbe5e}"
+        }
+    },
     "icons": {
         "48": "icons/icon-48.png",
         "96": "icons/icon-96.png"

options.js

@@ -3,7 +3,7 @@ function saveOptions(e) {
     e.preventDefault();
 }
 function restoreOptions() {
-    browser.storage.local.get('days', function(res) {
+    browser.storage.local.get('days').then((res) => {
         document.querySelector("#days").value = res.days || 0;
     });
 }