config.exs

# This file is responsible for configuring your application
# and its dependencies with the aid of the Mix.Config module.
#
# This configuration file is loaded before any dependency and
# is restricted to this project.
use Mix.Config

# Configures the endpoint
config :hello_phoenix, HelloPhoenix.Endpoint,
  url: [host: "localhost"],
  root: Path.dirname(__DIR__),
  secret_key_base: "ur8yC0kQtT36WAwO/uORZTkCMRcnJ9UZMSHDgaLYnpkFHBy86a/8x2TeO7Lb4bga",
  render_errors: [accepts: ~w(html json)],
  pubsub: [name: HelloPhoenix.PubSub,
           adapter: Phoenix.PubSub.PG2]

# Configures Elixir's Logger
config :logger, :console,
  format: "$time $metadata[$level] $message\n",
  metadata: [:request_id]

# Import environment specific config. This must remain at the bottom
# of this file so it overrides the configuration defined above.
import_config "#{Mix.env}.exs"

# Configure phoenix generators
config :phoenix, :generators,
  migration: true,
  binary_id: false

  #config :hound, driver: "phantomjs"
config :hound, driver: "selenium"
config :hound, browser: "firefox"

  # Start with default driver at port 4444 and use firefox
#config :hound, port: 4444, browser: "firefox"

# Define your application's host and port (defaults to "http://localhost:4001")
config :hound, app_host: "http://localhost", app_port: 4000

sample_test.exs

defmodule HelloPhoenix.SampleTest do
  use HelloPhoenix.ConnCase
   ExUnit.start
  # Import Hound helpers
  use Hound.Helpers

  # Start a Hound session
  hound_session
  IO.puts "sample test"

  test "GET /" do
    navigate_to "/"
    assert page_source =~ "Welcome to Phoenix"
  end
  
  IO.puts "after test block"
end

• Can you add your ConnCase?
• Check if your test_helper.exs already has ExUnit.start and remove it from your test file
• Keep just one hound config, if you need to incluse other options just add it for the single config:

# Try just `config :hound, driver: "selenium"` first
config :hound, driver: "selenium", browser: "firefox"