Address already in use for headless browser

I use Watir and headless Firefox/Chrome to scrape a website deployed with Hatchbox:

def perform(uid_number)
pool = RandomPort::Pool.new #random_port gem to generate a free port
port_number = pool.acquire #random_port gem
@company = Company.find_by!(uid: uid_number)
Webdrivers.configure do |config|
config.proxy_addr = "some_url"
config.proxy_port = "80"
config.proxy_user = "user_name"
config.proxy_pass = "password"
end
args = ["--headless", "--marionette-host=#{port_number}"]
browser = Watir::Browser.new :firefox, headless: true, options: {args: args}
#code for scraping goes here
browser.close
pool.release(port_number) #random_port gem
end

When I run above command from the command line, it works without problems. When I run several headless browsers (Firefox/Chrome) as a background job, I get Errno::EADDRINUSE: Address already in use - bind(2) for [::]:port_number (hence the use of the random_port gem). I read that the webdrivers should look for a free port, so I don't know how to fix this. Even when I kill the process, restart the server I get the problem after a short while. How can I solve this?

I would also be interested what setup you use to scrape JavaScript heavy websites as I'm just starting out. Thanks!