Watir does not support Firefox any more (you may still can get JSSH
working up to Firefox 4), so to run tests against Firefox, use
Selenium-WebDriver (aka Selenium 2).
You can find out more about running Selenium-WebDriver tests in
TestWise 2 in Zhimin's ebook at
http://zhimin.com/en/books/pwta
Chapter 12: Selenium-WebDriver