extjs version 2.2.1 error in firefox 4

I am using the extjs library t开发者_开发问答o implement a list view. This has been working for several years however however upon upgrading to Firefox 4 it quit working. In Firefox using firebug I am getting the following error: d is null in console.

It still works in ie7 & ie8 so I'm trying to find out if I just simply need to upgrade my version of extjs or if it is a configuration error on my part

Thanks