Firefox error lang/en.js on custom

I created a pretty simple custom for FCKEditor and it works great in both IE and Firefox - but I get a pop up "Error loading (mypath)/(mycustom)/lang/en.js"

But if I add a (blank) en.js, the error goes away, but FCK no longer works at all for many things in IE.

I have a directory for my custom, a GIF file named properly, and my fckplugin.js is as follows - again, it actually WORKS fine, just that silly error in FireFox only. And I can't find documentation on how to construct a lang/en.js that would work...

Thanks in advance,
-Rich

my line in the settings file:

FCKConfig.Plugins.Add( 'plugin_uploadfile', 'en', '/WebAdmin/FCKcustoms/' ) ;


contents of: FCKcustoms/plugin_uploadfile/fckplugin.js :

var plugin_uploadfileCommand=function(){
};

plugin_uploadfileCommand.GetState=function() {
return FCK_TRISTATE_OFF;
}

plugin_uploadfileCommand.Execute=function() {
window.open('/WebAdmin/FileList.aspx?UploadOnly=Y&dir=' + parent.document.forms.Form1.fileshareDir.value + '&Message=Choose a file to upload. The file will be added to your page automatically.', 'uploadFile', 'width=800,height=600,scrollbars=no,scrolling=no,location=no,toolbar=no');
}

FCKCommands.RegisterCommand( 'plugin_uploadfile', plugin_uploadfileCommand ) ;

var oplugin_uploadfileItem = new FCKToolbarButton( 'plugin_uploadfile', 'Upload File' ) ;
oplugin_uploadfileItem.IconPath = '/WebAdmin/FCKcustoms/plugin_uploadfile/plugin_uploadfile.gif' ;
FCKToolbarItems.RegisterItem( 'plugin_uploadfile', oplugin_uploadfileItem ) ;