Installed Nucore today, and i got the same problem with firefox after installation.
Starting Ubuntu from cd-rom, ?visitor?-user, won't work, since you don't have the permission to create or change anything on the hd.
This is how i fixed it;
- Download the .tar file on another pc, copy it on a USB.
- Boot Nucore, press F1, put; /etc/init.d/gdm start in command, press enter. User name; root, pass; nucore.
- connect USB to pc
- start from page 26 in the manual, you will find the "downloaded" file on the USB-drive.
After download, it seems that the Nucore.tar.tar contains another compressed file; Nucore.tar, this is the file you need to copy in Ubuntu and extract!
(super easy, but for people who don't got any pc-skills, and follow the guide, it's a usefull help)