I have successfully installed Snow Leopard using LifeHacker.com's method.
1. USB Thumb Drive partitioned and restore DMG file.
2. boot USB thumb drive and install snow leopard
3. Installed Mozilla Firefox and Yahoo Messenger
4. Reboot
5. Mozilla Firefox and Yahoo Messenger are now missing.
6. Installed them again, reboot
7. They're gone again.

What could have causes this?