Issue 6496 - Account for browsers that don't support notifications with buttons

lib/notificationHelper.js

Index: lib/notificationHelper.js
===================================================================
--- a/lib/notificationHelper.js
+++ b/lib/notificationHelper.js
@@ -206,14 +206,21 @@
         // from closing automatically.
         priority: 2
       };
-      browser.notifications.create("", notificationOptions, () =>
+
+      // Firefox and Opera don't support buttons. Firefox throws synchronously,
+      // while Opera gives an asynchronous error. Wrapping the promise like
+      // this, turns the synchronous error on Firefox into a promise rejection.
+      new Promise(resolve =>
       {
-        // Opera does not support the addition of buttons to notifications.
-        // Question type notfications always include buttons.
-        if (browser.runtime.lastError && activeNotification.type != "question")
+        resolve(browser.notifications.create(notificationOptions));
+      }).catch(() =>
+      {
+        // Without buttons, showing notifications of the type "question" is
+        // pointless. For other notifications, retry with the buttons removed.
+        if (activeNotification.type != "question")
         {
           delete notificationOptions.buttons;
-          browser.notifications.create("", notificationOptions);
+          browser.notifications.create(notificationOptions);
         }
       });
     }