Example: setting a http and https proxy for Firefox

profile = Selenium::WebDriver::Firefox::Profile.new
profile.proxy = Selenium::WebDriver::Proxy.new :http => 'my.proxy.com:8080', :ssl => 'my.proxy.com:8080'
browser = Watir::Browser.new :chrome, :profile => profile

Example: setting a http and https proxy for Chrome

switches = '--proxy-server=my.proxy.com:8080'
browser = Watir::Browser.new :chrome, :switches => switches