Gnome article on same topic: …gnome.org/…/understanding-gnome-shells-focus-ste…

You can test the more strict focus stealing prevention on Gnome with: gsettings set org.gnome.desktop.wm.preferences focus-new-windows ‘strict’.

And to unset it: gsettings set org.gnome.desktop.wm.preferences focus-new-windows ‘smart’

Firefox should also now have proper support for it in 141, but I think for Gnome you might need to wait for a bug fix in Gnome 49: