Why does the active Firefox window occasionally loses focus?

Occasionally, when I transition from one tab to another, the Firefox window loses focus. This is especially notable when I switch or close tabs via keyboard shortcuts. For example, I can be closing a bunch of tabs with Ctrl+w and suddenly it stops working. I have noticed that the Firefox window title is grayed out, indicating that the window has lost focus. Alt tabbing or clicking the window regains focus, but this is disruptive to the browsing experience.

I used xdotool getwindowfocus getWindowname to print the name of the active window to stdout every 20 s. When Firefox loses focus, nothing is printed, indicating that there is no other application that steals the focus from Firefox. Firefox is also the only application that displays this behavior.

I am on Firefox 47.0 on Antergos (Arch derivative) Linux with keel 4.6.2.1 64-bit. Why does this happen and what can I do to remedy the situation?

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