Travis CI supports Xvfb and Firefox. Travis offers documentation on leveraging these technologies in its CI environment.
In short, Xvfb can be spun up in advance of test execution. web-component-tester tests can be run in Firefox.
language: node_js node_js: - "0.12" before_install: - "npm install -g bower" - "bower install" - "npm install -g web-component-tester" - "export DISPLAY=:99.0" - "sh -e /etc/init.d/xvfb start" script: "wct"