to avoid inconveniences that might be caused by the overuse of the bundled key you might want to get your own key at plurk.com/API

then in firefox, type in the address bar: about:config ; proceed in spite of the warning, search for extensions.fireplurk.apiKey

by double clicking this line you can enter your own key to be used in your "instance" of the extension

and that way it's unlikely that you will exceed the daily 50.000 calls limit