I was unable to reproduce the issue in an fresh installation of Trisquel 6.0
i686 on another system with the following hardware:
AMD E-350 board with 2x1.6 GHz
8 GB RAM
GeForce 9500GT
Trisquel x686 installed on 8GB USB Stick
3D acceleration via Novueau worked in the fresh installation as well as after
the update. System had on-board GPU enabled. The issue seems to be related to
my hardware. I will see if this is related to older GeForce cards if I update
the system of my daughter (NForce 2 motherboard with onboard GPU plus
dedicated GeForce 4400.