Running selenium automation tests locally requires a browser (safari, firefox, chrome, internet explorer, phantomjs, etc) and sometimes a browser plugin or external shim binary. You will need to complete one or more of the below installations depending on which browser/s you want to automate.

By far the quickest way to get a test to run locally is by installing phantomjs.

Mac folks will find that "brew install phantomjs" works quite well to install phantomjs. Just make sure the installation is somewhere on your $PATH environment variable.

If SSL certificate is untrusted, phantomjs may timeout loading your application. In such scenarios, you could pass [driver capabilities] (https://github.com/paypal/nemo#servercaps-optional) to nemo like below,

{
 "driver" : {
    "browser" : "phantomjs",
    "serverCaps" : {"phantomjs.cli.args": [ "--ignore-ssl-errors=yes", "--ssl-protocol=tlsv1"]}
 }
}

All that is necessary in order to use Firefox is that you've got an appropriate version of the Firefox browser for the version of selenium-webdriver that nemo is currently using. Sometimes, you may need to forego a Firefox update in order to ensure that your nemo test environment continues to work.

To automate the Chrome browser, you will need the chromedriver binary visible in your computers $PATH

Find the latest here: http://chromedriver.storage.googleapis.com/index.html

For OS X, if you're using Homebrew, there's a Homebrew formula, which makes installation much easier. To install, it's simply brew install chromedriver.

As of this edit, there is an issue where the latest selenium-webdriver client supports Safari, but does not support Chrome or Firefox. Until this situation is resolved, and unless automating Safari is a REQUIREMENT, avoid it.

There are other "drivers", like Appium or ios-driver, which can be used to automate iOS/Android simulators and devices. That setup is covered elsewhere.