env.rb

module Firebug
  VERSION = "1.6.2"
end

Capybara.register_driver :selenium_with_firebug do |app|
  require 'selenium/webdriver'
  
  profile = Selenium::WebDriver::Firefox::Profile.new
  profile.add_extension("spec/support/firebug-#{Firebug::VERSION}.xpi")
  profile["extensions.firebug.allPagesActivation"]  = "on"
  profile["extensions.firebug.net.enableSites"]     = true
  profile["extensions.firebug.console.enableSites"] = true
  profile["extensions.firebug.script.enableSites"]  = true
  profile["extensions.firebug.currentVersion"]      = Firebug::VERSION

  Capybara::Driver::Selenium.new(app, :profile => profile)
end

Capybara.javascript_driver = :selenium_with_firebug

Save the Firebug XPI to spec/support/firebug-#{Firebug::VERSION}.xpi

This is for Capybara version ~> 1.

For Capybara version < 1, change line 16 to Capybara::Selenium::Driver.new(app, :profile => profile).