Does the ReadyNAS 104 support 6TB drives

There are no 6TB drives on the compatibility list for your NAS: http://kb.netgear.com/app/answers/detail/a_id/20641

If you want to use 6TB disks I would recommend using the 314.

Whilst you could use 6TB disks in the 104 do note that the 104 does not support volumes with a larger capacity than 16TB. You can create multiple volumes, but each volume must be less than 16TB in capacity.

That sucks.

Is there a firmware update in the pipe for the 104 to support 6TB drives

The 16TB volume capacity limit is a hardware limitation.