Add option to override Firefox binary used in system tests

test/application_system_test_case.rb

@@ -9,6 +9,7 @@ end
 class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
   driven_by :selenium, :using => :headless_firefox do |options|
     options.add_preference("intl.accept_languages", "en")
+    options.binary = Settings.system_test_firefox_binary if Settings.system_test_firefox_binary
   end
 
   def before_setup

test/teaspoon_env.rb

@@ -100,10 +100,12 @@ Teaspoon.configure do |config|
   # Capybara Webkit: https://github.com/jejacks0n/teaspoon/wiki/Using-Capybara-Webkit
   require "selenium-webdriver"
   config.driver = :selenium
+  firefox_options = Selenium::WebDriver::Firefox::Options.new(:args => ["-headless"])
+  firefox_options.binary = Settings.system_test_firefox_binary if Settings.system_test_firefox_binary
   config.driver_options = {
     :client_driver => :firefox,
     :selenium_options => {
-      :options => Selenium::WebDriver::Firefox::Options.new(:args => ["-headless"])
+      :options => firefox_options
     }
   }