Disabling JavaScript when using Capybara + Selenium

I have an app that's designed to still be functional when JavaScript is disabled, so I wanted to write some specs that covered those cases.

I'm using Selenium (Firefox) with Capybara and I'm registering an new driver with JavaScript disabled (via Selenium's javascript.enabled property)

# spec/rails_helper.rb
Capybara.configure do |config|
  config.ignore_hidden_elements = true
  config.default_driver = :selenium
end

Capybara.register_driver :disable_js do |app|
  profile = Selenium::WebDriver::Firefox::Profile.new
  profile["javascript.enabled"] = false
  Capybara::Selenium::Driver.new(app, profile: profile)
end


# spec/features/siging_in_spec.rb
context "JavaScript disabled", driver: :disable_js do
  it "user can still sign in" do
    # ...
    # ...
  end
end

The feature specs are failing to actually disable JavaScript. When the browser window pops up during testing and I pause it with binding.pry, I can definitely click around on items I know require JavaScript and see them working.

Side note: If I actually go to my Firefox settings and disable JavaScript, the test passes. So it appears it's inheriting whatever configuration I set in my browser, and not actually using the configuration specified when registering the driver.

Is this the correct approach here, or is there something I missed?

Thanks!