If Firefox shows you a "Warning: Unresponsive plugin" prompt, it means that a plugin, like Flash, is taking longer than expected to do what you requested. This article covers some solutions and workarounds.
If you get this warning once, you can try to click Continue to give the plugin more time to complete your request. This may not always be successful. If you get this warning a second time you should probably click Stop plugin to make the plugin stop immediately (crash).
Here's what to try if you get this warning often:
If you don't want to be warned when a plugin hangs, you can disable this warning by checking the "Don't ask me again" checkbox. When you do that, Firefox will allow the plugin to hang for up to 45 seconds before it crashes.
If you change your mind and want to re-enable the warning, do the following:
In the address bar, type about:config and press EnterReturn.