Selenium 3.0 requires the geckodriver tool when running Selenium tests with Firefox. Geckodriver is a library that translates WebDriver calls for Mozilla's Marionette driver.
It was pretty easy to get this working on travis - you can just download the geckodriver tool and put it in the PATH, like this:
language: python addons: firefox: "45.4.0esr" python: - "2.7" before_install: - wget https://github.com/mozilla/geckodriver/releases/download/v0.11.1/geckodriver-v0.11.1-linux64.tar.gz - mkdir geckodriver - tar -xzf geckodriver-v0.11.1-linux64.tar.gz -C geckodriver - export PATH=$PATH:$PWD/geckodriver