With Firefox 48+ using Gecko engine, you’ll need to install geckodriver in order to use Selenium.
You can install geckodriver here, or if you’re on Mac, you can install it with homebrew:
brew install geckodriver
If you installed without homebrew, you’ll need to make sure that the executable is found in the system path:
echo 'export PATH=$PATH:/path/to/geckodriver' >> ~/.bashrc
Now you can launch a Firefox browser with Selenium.