In Firefox with the Firebug console open, you probably get “Permission denied to call method Location.toString”. This error will get thrown on any page that has a Flash file inside an iframe. The error is prevalent because many ROS ads (Doubleclick, etc.) are served in this exact way. Adobe has a JIRA bug open that they have marked with the absolute lowest priority possible (none) because “the error is cosmetic. Exposes no functional or security issues.”
The only solution/workaround is to avoid putting any iframes with Flash on your page. This solution doesn’t work for many developers who have no control over the way ads are served, but Adobe claims it isn’t really a problem.