Using Extjs in Firefox Extension

I'm developing a Firefox extension, and need to work with an ExtJS component on a page. Has anyone been able to do this? I've loaded the ExtJS Javascript files in my XUL, but doing something like Ext.getCmp('my-button').disable() has no effect. Do I need to somehow configure the Ext ob开发者_JAVA百科ject with the document object? Any other suggestions?

Just a shot in the dark but I know when I wrote Selenium testing a while back I had to use window.Ext.getCmp('my-button'). maybe that will work.