This add-on deactivates new plugins after installation. By installing new software, you may unintentionally install a new plugin. If that occurs and if the preference 'plugin.default.state' is set to 2 (preference accessible through about:config), Firefox will activate that plugin. As a result, you may experience unnecessary security risks and possibly decreased performance. This extension prevents both potential consequences by deactivating the new plugin after it has been installed. Additionally, it notifies you about the deactivation.
Note: New Plugin Disabler probably does not deactivate the OpenH264 Video Codec plugin included since Firefox 33.