If you have EwmhDesktops configured (including by using XMonad.Config.Desktop as your base config, or one of its descendants such as XMonad.Config.Xfce or XMonad.Config.Gnome) then we handle the EWMH focus request message.

Many people do not like it when their browser demands focus (Firefox and Chrome are both happy to demand focus when some other program has focus, instead of only when they are switching focus from one of their windows to another), so we do not force support for the EWMH focus change message by default as this would make it very difficult to turn off.