In this specific case of what Mozilla is trying to achieve, they are researching the topic and looking for help. Mozilla doesn't sound like they want to ban all overlays, just the obnoxious ones.
When I think "action bars", I tend to think of banners that expand into a flexbox row or rows of my choosing and I don't overlay anything. If it is meant to hide a part of the UI, it's better to hide or replace that UI explicitly rather than cover it up graphically in an overlay.
Another reason to explicitly hide/replace UI instead of covering it graphically is because that doesn't stop someone from interacting with that covered UI, it just makes it more irritating to do so. I'm not just talking about Dev Tools and similar "cheat" options, but not every user is necessarily interacting with your site or application with sight/graphically. An overlay doesn't stop a screen reader, for instance. (A lot of the modal popups that Mozilla is hoping to block are entirely ignored by a browser's "Reader Mode", which is sometimes my first attempt when I see one.)
But an overlaying action-bar with a "confirm" button that otherwise allows you to scroll before you delete would be a great middleground.
But how can a browser tell the difference?