Enable capybara tests with Firefox (via geckodriver)

To run capybara with firefox, set the ENV variable:
`WEBDRIVER` to `firefox`

It will run in 'headless' mode by default unless
`WEBDRIVER_HEADLESS` is set to `false`, `no` or `0`

spec/support/capybara.rb

@@ -79,8 +79,30 @@ Capybara.register_driver :chrome do |app|
   )
 end
 
+Capybara.register_driver :firefox do |app|
+  capabilities = Selenium::WebDriver::Remote::Capabilities.firefox(
+    log: {
+      level: :trace
+    }
+  )
+
+  options = Selenium::WebDriver::Firefox::Options.new(log_level: :trace)
+
+  options.add_argument("--window-size=#{CAPYBARA_WINDOW_SIZE.join(',')}")
+
+  # Run headless by default unless WEBDRIVER_HEADLESS specified
+  options.add_argument("--headless") unless ENV['WEBDRIVER_HEADLESS'] =~ /^(false|no|0)$/i
+
+  Capybara::Selenium::Driver.new(
+    app,
+    browser: :firefox,
+    desired_capabilities: capabilities,
+    options: options
+  )
+end
+
 Capybara.server = :puma_via_workhorse
-Capybara.javascript_driver = :chrome
+Capybara.javascript_driver = ENV.fetch('WEBDRIVER', :chrome).to_sym
 Capybara.default_max_wait_time = timeout
 Capybara.ignore_hidden_elements = true
 Capybara.default_normalize_ws = true