Mozilla is changing the internals of Firefox in a way that makes it more stable and secure, but which also makes the community-provided Firefox Driver no longer work. As such, if you use Firefox for your testing, you'll need to use the geckodriver, which is an executable similar to the chromedriver and MS's edgedriver. You'll need to start using geckodriver even if you're using Selenium 2
https://saucelabs.com/blog/selenium-3-is-coming