Adding your comment from the Pivotal Blog here as reference material:
There's two ways to do what you want using WebDriver APIs:
Use Profile#add_extension. This lets you programatically add an .xpi to the profile used by WebDriver. Here's an example:
Use the same approach as above, but tell WebDriver explicitly that it should use the named profile as a template, i.e. if the profile was named "capybara":
Selenium::WebDriver.for :firefox, :profile => "capybara"
Hooking this into Capybara should be simple using the new API to configure drivers:
Great info here: http://waltersmith.us/articles/using-firebug-with-capybara
@eirc: That post has an "ignore this post" update at the top. The info about patching the XPI is pretty bad, since the same thing can be done by simply setting a preference:
profile['extensions.firebug.currentVersion'] = "9.99"
Warning blindness has surely set in...!
Sorry for the miss-information and thanks for the info.