Mozilla is working on a way of killing unresponsive plugins rather than having to wait until the browser kills them automatically. Chrome already has a similar feature and it would be a welcomed addition to Firefox.
If a plugin, such as the Flash Player, hangs or becomes unresponsive for whatever reason, Firefox will ask the user whether to terminate the plugin, or wait for it to finish whatever it is it's doing.
In most cases, when a plugin becomes unresponsive, something went wrong and it won't recover.
Currently, Firefox also becomes unresponsive during this time and waits 45 seconds before killing the plugin and ending the process that was using up the CPU.
Starting with Firefox 19, the browser includes a mechanism and the necessary UI to stop the plugin ahead of the regular 45 seconds. The feature is only planned for Windows and may be pushed to Firefox 20 or beyond if it's not ready in time.