You also need to go to the correct path to the local host:
http://localhost:8080/components/polymerfire/demo/
So, the full procedure (from the command line):
-, polymer-cli , .
npm install -g bower
npm install -g polymer-cli
.
shell.sh
git clone https://github.com/firebase/polymerfire.git
cd polymerfire
bower install
polymer serve
open http://localhost:8080/components/polymerfire/demo/