Receiving this error could mean that the visitor clicked "X" on the permission prompt asking the user if they'd like to receive notifications from your website.

On Firefox versions 52 and older, this message will appear if the visitor clicks anywhere on the website before interacting with the prompt.

How do I un-do this, and allow Web Push permission for my site?

Web Push is always opt-in in Firefox. A site cannot send you push messages without your permission. If you accidentally denied the permission request, please follow these steps to allow notifications for your site:

1. Click the menu button  and select Preferences.
2. Click Privacy & Security from the left pane.
3. Scroll to the Permissions section.
4. Click the Settings… button next to Notifications.
5. Select your website.
6. Set the Status to "Allow".
7. Make sure "Block new requests asking to allow notifications" is unchecked.
8. Click the Save Changes button. 

   

Source: http://mzl.la/20jwSoU