I would suggest trying the Advanced User Actions API that was added in the 2.0rc1 release yesterday, as it looks like you’re using the Selenium 1 API still (going through WebDriverBackedSelenium), and I’m not sure how much Firefox 4 support that provides. I’m not using Java for my Selenium tests, but it looks to me like what you would want to do is something like this:
Actions builder = new Actions(driver); // Or maybe seleniumDriver? Not sure which one to use
Actions hoverOverRegistrar = builder.moveToElement(menuRegistrar);
hoverOverRegistrar.perform();