I can't see the screenshot I attached to the original support request, so here it is.

As far as my understanding is you can have all sites notify or none. Why you get so many notifications will be a continuing issue. The x is there to do away with the notification and closing them will allow you to see the screen. It is a slack issue with amount of notifications they send. Firefox is doing what it should do.

So decide if you want to click them away 1 at a time till can see the screen or get none at all.

I understand that Firefox is doing what it should do, I am hopeful for the ability to customize this somewhat. Slack is a fairly common web application, and while I agree that it would be nice to handle this within Slack, that's not possible at present.

Clicking them to close isn't a great solution. Notifications disappear on their own after 5 seconds. Clicking to close initiates a 2 second fade out, which is completely unnecessary as I have already indicated I don't want to see this notification. Even removing this fade out animation would be nice, but currently there is no way to configure notification display at all. Again, I am happy to do this via an about:config entry, browser CSS or even a plugin if this exists. To the best of my knowledge, it does not.

Hi, seems you may find something here : https://addons.mozilla.org/en-US/firefox/search/?platform=windows&q=slack

Please let us know if this solved your issue or if need further assistance.

This doesn't solve my problem.