Is there a proved mouseOver workaround for FirefoxDriver in Selenium2?

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();

I use this code to get a mouseover event for a specific webelement. It does not need native events.

protected void mouseOver(WebElement element) {
    String code = "var fireOnThis = arguments[0];"
                + "var evObj = document.createEvent('MouseEvents');"
                + "evObj.initEvent( 'mouseover', true, true );"
                + "fireOnThis.dispatchEvent(evObj);";
    ((JavascriptExecutor) driver).executeScript(code, element);
}

Actions action = new Actions(driver);
action.moveToElement(element).build().perform();
action.moveByOffset(1, 1).build().perform();