KDE does have focus stealing prevention, Have you messed with that at all? https://docs.kde.org/stable5/en/kwin/kcontrol/windowbehaviour/index.html#focus-focusstealing The only thing I can think of is maybe some notification steals the focus and doesnt give it back to the window.

https://xkcd.com/1172/

Facebook was the real gateway drug this whole time.

We have a coffee machine that runs on linux.

Does the facebook marketplace have an onion link?

The firefox reader mode button doesn’t show up for me on that site. I wonder if its just a poor site or if they are intentionally trying to break it.