Next time you see it, could you get the exact wording of the message? I wonder whether it's a variation on the message described in this article: [[Warning Unresponsive script - What it means and how to fix it]]. Generally speaking the scripts run by most sites are handled internally by Firefox without need for any plugins. The Java plugin was used to run "Applets" and most sites do not use those (old games on Pogo being one exception).