I took the code from the portal example of preview 5. I modified it, so that the portal extends from Ext.Panel instead of Ext.Viewport. I think that shouldn't be a problem.

When I create this portal (then it gets rendered into a view port) Firefox tells:
[Ext.Loader][not enabled] Missing required class: Ext.app.Portal
. IE 9 creates and renders it perfectly without any error. What Am I doing wrong?

Does it have something to do with MVC 3 (VS 2010) that I'm using? I also tried this within a normal aspx page. This works correctly in both browsers.

I attached the solution if you are interested in this 'issue'.
/Views/Shared/_Layout.cshtml contains all the script referencing, including the final "page script" which creates the portal.

/Home is the MVC page that only works in IE.
/WebForms1.aspx is the default aspx page that works in FF, too.

I removed the ExtJS folder from the project due to file size. It's the original ExtJS download that I included into the project.