If you install software in Trisquel with apt-get is going to be really hard
that you install proprietary software.
But if you use PPA's of Ubuntu this are not officialy supported by Trisquel
and you have to check to make sure your freedom is not at risk.
If you are a programmer or you like to play with the system there is a chance
you will be asked to install software from python with pip
pip install package
In that case or some other package manager I think npm on JavaScript, (I
don't know much about this package managers) but you have to be careful about
them. Most of them don't have a strong policy about free software. So even in
Trisquel you could install non-free software.
Something similar happens to program's plugins. Just because a program is
free doesn't mean the plugins are, so be careful if you download a plugin
from Internet.
This also is applied to Firefox and Thunderbird plugins, even if MPL 2.0 is
compatible with GPL the plugins they recommended are not necessarily Free
Software.
Also, if you use Emacs or you ever starhmusing Emacs you probably will be
suggested to install MELPA (more pthatns). I don't know about Vim but it also
has lots of plugin's.
Bottom line Trisquel will protect your freedom with apt-get.
Apart from that, you are responsible of making sure other software you
install is also libre. Which will probably be though.
Except from Firefox and Thunderbird most of the cases I mentioned rarely have
non-free software.
