I think this case was the browser was active, but not the tab, so the browser reports that.

Many, many telemetry metrics have been added in the name of power and efficiency. If a page refreshes every 30 seconds, is it still worthwhile doing it when the tab isn’t active? It would be better to wait until the tab is active again, then refresh immediately.

That being said, all of these capabilities are a privacy nightmare, only increasing the precision of browser fingerprinting and user monitoring. Firefox could have taken a stance on refusing to implement them, but I don’t think it has an easy opt out.