When firstboot launches an X server, it specifies vt7 explicitly (line 152). If
for some reason an X server is already running on vt7 this can do very strange
things to the box, possibly locking it up. See bug#86576 for more details.
If you don't specify a VT number then X will take the first available and thus
play nice with any existing X servers.
Should be fixed in firstboot-1.0.9-1 in Rawhide.