ruby-on-rails - Watir - using proxy in Firefox with authentication

Question

I want to set up a proxy in Firefox with Watir. The proxy requires authentication - username and password.

I get error when I want to do the following thing:

profile = Selenium::WebDriver::Firefox::Profile.new
profile.proxy = Selenium::WebDriver::Proxy.new :http => "user:pass@133.212.109.502:8080"

The error is:

ArgumentError: invalid value for Integer(): "pass@133.212.109.502:8080"

What am I doing wrong? How to use proxy with authentication in Firefox properly?

Thanks for help