Selenium-rc: is there a way to change the user-agent without changing the Firefox profile

Question

I want to run tests that change the user-agent in the http request sent from the browser (like the FF add-on, user agent switcher does). I saw you can do it by playing with the FF profile (http://seleniumhq.org/docs/09_webdriver.html).

Is there a way to do it within a test? Something like the function addCustomRequestHeader() that sets a header rather than adding it