Trisquel 6 was using a libre version of Linux 3.2. If you are not afraid of
unpatched vulnerabilities (I do not think anybody is backporting fixes to
such antique kernels), I guess you could install a .deb of that version.
I do not know where to find Trisquel 6's deb packages. However, I found
http://archive.gnewsense.org/gnewsense/pool/main/l/linux/linux-image-3.2.0-4gnewsense1-amd64_3.2.57-3+deb7u1gnewsense1_amd64.deb
in gNewSense's repository. After downloading that file, double-clicking on
it should propose its installation.
After installing, reboot and; in the "Advanced options for Trisquel" (or
something like that) of GRUB, choose the newly installed kernel. If the
kernel suits you, you can then remove the newer kernels or, safer, keep them
but configure GRUB to boot version 3.2 by default. That is done through the
variable GRUB_DEFAULT in /etc/default/grub (you can write GRUB_DEFAULT=saved
and GRUB_SAVEDEFAULT=true to boot by default the last version that was
booted).
