Icecat is a perfectly acceptable substitute for Firefox. You can install
proprietary plugins on Icecat if you want (far be it from us to tell you what
you can install), but realize you won't get any assistance in installing
anything non-free, and we will not provide any support if you choose to do
so.
There is nothing inherent in Trisquel that blocks you from using proprietary
software. Freedom cannot be forced upon anyone (that would be an oxymoron).
But, again, realize that this community values the ethical principles behind
software freedom and believe it is a choice worth making. You are free to
choose what you want on your computer, but we as a community are free to
uphold our principles.
However, instead of asking for help in installing a certain piece of non-free
software, perhaps you could tell us what you hope to achieve with it, so that
we may suggest a free alternative.
FYI, we don't use free software just because proprietary software "rubs RMS
the wrong way." We respect RMS greatly for starting the free software
movement and launching GNU in 1983, which led to GNU/Linux (not just merely
"Linux") distros like Trisquel here. But we don't choose anything because he
tells us to. We choose free software ourselves because it is the ethically
and socially superior choice, and we hope you do too.
