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.