For those who come to this issue using Firefox and the above solutions don't work, you may try the code below (my original answer is here).
from selenium import webdriver
profile = webdriver.FirefoxProfile()
profile.DEFAULT_PREFERENCES['frozen']['marionette.contentListener'] = True
profile.DEFAULT_PREFERENCES['frozen']['network.stricttransportsecurity.preloadlist'] = False
profile.DEFAULT_PREFERENCES['frozen']['security.cert_pinning.enforcement_level'] = 0
profile.set_preference('webdriver_assume_untrusted_issuer', False)
profile.set_preference("browser.download.folderList", 2)
profile.set_preference("browser.download.manager.showWhenStarting", False)
profile.set_preference("browser.download.dir", temp_folder)
profile.set_preference("browser.helperApps.neverAsk.saveToDisk",
"text/plain, image/png")
driver = webdriver.Firefox(firefox_profile=profile)