Trisquel doesn't include nonfree software. A lot of websites, though,
silently install Javascript code into your browser. The way it works is the
website requests to load a Javascript file and the browser usually loads it
transparently, without telling the user what's going on.
Unfortunately, several websites depend on proprietary Javascript code to
operate correctly.
The only solution to this problem is to tell the browser to not run the
nonfree Javascript code requested by websites. The simplest way is to disable
Javascript in the browser itself, but this also disables free Javascript
code, so it creates more of a hassle. That leaves LibreJS and NoScript.
LibreJS attempts to filter automatically and only run free and trivial code,
but it's horribly imperfect and has weird bugs that mess with the browsing
experience in other ways. NoScript requires manual work.
So, short answer: if you want to deal with this problem the easy way, either
disable Javascript in the web browser or install LibreJS. Just note that many
websites will break. If you want better, more fine-tuned control over what
Javascript code is allowed to run, use NoScript and start by emptying the
whitelist (it whitelists some sites which host nonfree Javascript by
default). Unfortunately, there isn't a better solution at this time.
