Mythtv's configure script disables firewire support, even with the --enable-firewire flag set by the ieee1394 USE flag because it wants libavc1394-0.5.0 or later. Reported for 0.19-r1 in http://forums.gentoo.org/viewtopic-t-439145-start-0-postdays-0-postorder-asc-highlight-.html, but it looks like mythtv-0.19_p9163 is also effected.
Fixed in -r1 for a while.