Watir - using proxy in Firefox with authentication

Question

I want to configure a proxy server in Firefox using Watir. The proxy requires authentication - username and password.

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

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

Error:

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

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

thanks for the help