So this has been bothering me for a while now where I would sometimes get notifications on my desktop from reddit r/archlinux posts. I finally took some time to track down where they are coming from, and it looks like it's push notifications from firefox. I guess these come in even if you don't have those pages loaded in your browser, which is kind of confusing.
Anyways, this blog post explains how to turn them off, and there is an option to not allow them ever which is also handy (remove all websites and block new requests): https://blog.mozilla.org/firefox/no-notifications/