What version of Firefox are you using?  Are you sure that the version of WebDriver you have supports the version of Firefox?  Try updating to the latest version of WebDriver and a version of Firefox that you know is supported like 7.0.1.
Another possible solution is that you can start Firefox with any profile instead of the default one that ships with Selenium.

DesiredCapabilities cap = new DesiredCapabailities();

FirefoxProfile profile = new FirefoxProfile("path_of_profile");

cap.SetCapability("firefox_profile", profile.ToBase64String());

WebDriver driver = new FirefoxDriver(cap);