How do I use ruby-debug in Capybara w/ selenium
Setting Capybara.app_host = 'http://localhost:3000' and running script/server -e test in another process worked.
I don't have a real answer myself, but your question reminded me of this post:
Selenium doesn't work with Cucumber/Capybara (out of the box) - MacOSX
Where the questioner shows how he used ruby-debug to figure out why a missing dependency was helping selenium fail to open the browser.
Hope this helps!
I found this works (in the cucumber env.rb file)
Capybara.default_wait_time = 2000 #default: 2