Any ideas? I've tried setting webdriver.firefox.useExisting=true as described in the link below hoping to reuse an existing window with firebug enabled, but no such luck.
you have to create a specific profile where Firebug is always enabled, then launch firefox with this profile in your configuration
The quick answer is:
browser = Watir::Browser.new(:firefox, :profile => "default")
But if you are running tests you probably shouldn't use the default profile.
profile = Selenium::WebDriver::Firefox::Profile.new
profile.add_extension "/path/to/firebug.xpi"
browser = Watir::Browser.new :firefox, :profile => profile
来源：https://stackoverflow.com/questions/6065092/how-do-i-get-watir-webdriver-to-start-firefox-4-with-firebug