Webdriver - Can't use headless mode in Firefox when using preferences

This error message...

AttributeError: 'Options' object has no attribute 'add_preference'

...implies that an instance of Options doesn't supports the attribute add_preference.

add_preference is supported by an instance of FirefoxProfile() only.

You can find a detailed discussion to use add_preference with an instance of FirefoxProfile() in Python: Unable to download with selenium in webpage

So your effective code block will be:

from selenium import webdriver;from selenium.webdriver.firefox.options import OptionscProfile = webdriver.FirefoxProfile();dwnd_path = os.getcwd();cProfile.add_preference('browser.download.folderList', '2');cProfile.add_preference('browser.download.manager.showWhenStarting', 'false');cProfile.add_preference('browser.download.dir', 'dwnd_path');cProfile.add_preference('browser.helperApps.neverAsk.saveToDisk', 'application/octet-stream,application/vnd.ms-excel');options = Options()options.headless = Truedriver = webdriver.Firefox(firefox_profile=cProfile, firefox_options=options, executable_path=r'C:\path\to\geckodriver.exe')

Reference

You can find a detailed discussion on how to use the headless argument through an instance of Options() class in How to make firefox headless programatically in Selenium with python?