In bug 1559697 I noticed that notifications from the extension API did not have a Close button, and after reading the code I figured that the Close button only appears if a principal is passed to the alerts service.

Doing so had unwanted consequences (bug 1577316), so I'm reverting that change. It would still be nice to have a Close button anyway.

It's not obvious why the Close button is conditional on the existence of a web principal in the first place. It seems that the Close button should unconditionally be shown, so I'm going to attach a patch that does this.

Before this change, desktop notifications without content principal
would not have a close button, which prevents users from having an easy
way to dismiss the notification.

With this change, the close button always appears.

By MattN at https://phabricator.services.mozilla.com/D49747#2038737

Given that the current behaviour is normal for macOS I don't think it's worth taking this. I think this should be addressed by https://bugzilla.mozilla.org/show_bug.cgi?id=1225110 as we only have a limited number of actions we can reasonably show and the close button takes up one of those. A holistic design for macOS actions to support developer actions (buttons) is necessary IMO or we may just end up reverting this. Sorry

Since the close button is not going to be added, I'm closing this bug.