Just in case you have not noticed it yet, because you might run Firefox 45 ESR, Mozilla broke the Firefox extension that Selenium is using to drive it. This means, simple and painless automation with Firefox is currently only possible, if you either downgrade to FF 45 ESR or jump through some hoops.
You might not need to downgrade your browser locally or go the Marionette way yet, you can simple install FF 45 (unzip it only for your platform) and point to the binary using a system property. Pass this to your JVM at start or by overwritten it from the inside and you are good to go. Adjust this to your local platform needs.
-Dwebdriver.firefox.bin=<path to binary>/firefox