This won't harm you and you can just ignore it.

I'm not sure if there is a way to delete it.

Thanks ... I found the way to remove it in the following link:

http://kb.mozillazine.org/About:config

To reset a preference to its default value or to remove an added preference, context-click (right-click) on the preference and select Reset. If you added the entry via about:config, the preference will no longer be listed after restarting the program. (For more information about resetting preferences, see this article).