Tor-browser doesnt appear to be in portage, but you can just download it, extract the tarball and run it (no compilation/installation required).
https://www.torproject.org/projects/torbrowser.html To run it without the tor network: "Upper right hand side of Tor Browser click on the three horizontal bars, click on Options, select the Advanced tab, click on Network, click on Settings, select "No Proxy" and hit OK. Then type "about:config" into the url bar, go to "network.proxy.socks_remote_dns" right click and select Toggle [to set it to false]' (From http://superuser.com/questions/1117383/can-i-use-tor-browser-without-using-tor-network ) Then you can check out how unique you are with sites like; https://panopticlick.eff.org/ https://www.amiunique.org For me, Firefox (+plugins) came out better than Tor browser. Firefox is let down by User Agent (0.31%), and Tor browser is let down by Screen Resolution (<0.1%, However, its reporting window size, which will change a lot).
