You could try an old version of gNewsense 3.0 or even v.2. I would imagine
an older version of Trisquel with net install would work as well. Just pick
the bare minimum to run the system.
Crunchbang Linux is an option as well, it is based on Debian Stable, but you
will have to disable the non-free repos, as they are active by default with
Crunchbang. That would make it technically Libre.
If you are comfortbale with terminal, you could ditch a DE and WM altogether,
that would be the lightest option.
