I learned how to use Firefox 4 with watir and webdriver (on Win7 x64), setting profile items. Example:
profile = Selenium::WebDriver::Firefox::Profile.new
pr
In Firefox 6+, I couldn't get this to work without specifically setting the 'browser.download.folderList' value:
profile = Selenium::WebDriver::Firefox::Profile.new
profile['browser.download.folderList'] = 2 #custom location
profile['browser.download.dir'] = download_directory
profile['browser.helperApps.neverAsk.saveToDisk'] = "text/csv, application/csv"
b = Watir::Browser.new :firefox, :profile => profile
See: http://watirwebdriver.com/browser-downloads/