I've seen several mentions of an issue when running tests in Firefox: the Flash player doesn't load the SWF if the browser window doesn't have focus. This causes sporadic test failures if the system used for running tests is doing anything else. I've verified that if the Firefox browser is behind IE the test will hang until it times out or else I bring Firefox to the foreground manually.
A workaround that allows the Selenium2/WebDriver test to bring the browser to the front or to override this behavior of the player would be much appreciated.