Given the age of your card (it was launched in 2008 according to Wikipedia),
it is unlikely that the newest Mesa is required.
Try to install the package named libgl1-mesa-dri-experimental (you can, for
instance, search it in the Synaptic package manager, which can be launched
from the System/Administration menu). It will probably work from a Live CD
too. However you may need to restart the X Server (without rebooting!). To do
so you can log in a "real" terminal with Crtl+Alt+F[1-6] (i.e., Ctrl+Alt+F1
or Ctrl+Alt+F2 or...) and then execute this command to restart Trisquel's
display manager:
$ sudo service gdm restart
The 3D acceleration brought by libgl1-mesa-dri-experimental comes at the cost
of less stability (hence the "experimental" in the name).
