I don’t think you can do that via userChrome.css or userContent.css. It is native anonymous content - kind of similar to scrollbars. They aren’t part of the page so you can’t use userContent.css but they are not chrome either so userChrome.css won’t work. And like scrollbars, you would need to have agent mode stylesheet to modify it so practically you would need to use autconfig to inject and agent mode stylesheet with this content:

:-moz-native-anonymous .paused-dbg-root[overlay]{
  display: none;
}

I mean, if there is some totally different way to remove the overlay then you could use that, but if you plan to do it with CSS then that’s the route you need to go.