Tor

You does not need to use Firefox(Tor Browser Bundle) in order to use the onion router. I am using Tor already for around one year and have it always turned on, even now. Assuming you are using Trisquel 5.0 with Abrowser, just follow my installation procedure described here:

Open Terminal (with [Ctrl]+[Alt]+[T]), then write:
$ sudo add-apt-repository 'deb http://deb.torproject.org/torproject.org natty main'
$ sudo apt-key adv --keyserver pgp.mit.edu --recv-keys 886DDD89
$ sudo apt-get update;sudo apt-get install vidalia polipo
$ A=vidalia.desktop;ln -s /usr/share/applications/$A ~/.config/autostart/$A

Now, install Torbutton and reboot.

You can check if your browser is configured properly by clicking here.

I also recommend you to use 'Peerguardian Linux' which blocks the access of anti-P2P organizations to your PC.