Strange message in terminal when visiting a website
You are getting that message because you launched Firefox from the command line.

I assume that you used &amp; to put Firefox into the background but that doesn't redirect the output so any error will get written to your terminal session. To stop that appearing you need to add 2&gt; /dev/null before the &amp;.

See this answer for more :

https://stackoverflow.com/questions/18012930/how-to-redirect-all-output-to-dev-null

As for what the error means I assume that website has run some Javascript that hit Firefox's security system and bounced.