This is possibly due to having the built-in Tracking Protection enabled in Firefox. My apologies for not mentioning this limitation in the overview (it has now been added) but this was discovered by another user a while ago. I don't know of any way to fix the problem in the add-on itself so unfortunately it comes down to which feature is more important to you. If you want to have both features, you might try to replace the built in Tracking Protection with an add-on providing similar functionality and see if that works with RedditNotifier.