Looks like we can't reproduce the issue or there's a problem in the test case provided.
Wrong cursor over file upload button, using Firefox
There seem to be a bug while hovering the "Browse" button of a FileUploadField object : The wrong cursor is displayed. It's supposed to be a hand cursor ("cursor: pointer"), but it's a text-selection cursor ("cursor: text").
We can see this bug here : http://www.sencha.com/examples/#ExamplePlace:fileupload
I'm using Firefox 17.0.1 on Sencha GXT 3.0.1.