[Update – this can (was for me) still be a fix for this issue in 2015 |mdurrant|]
I came across this problem lately.
You should upgrade to capybara v1.0.1 to have a correct selenium webdriver.
To be sure I added:
gem 'selenium-webdriver', '2.25.0'
in my Gemfile.
Important note:
The selenium-webdriver gem is updated, and a new version released, for every subsequent version of Firefox. Presently, version 2.25.0 is needed to support Firefox 15.