This seems to be a JavaScript issue which only occurs on Firefox. I am currently investigating this and will fix it for the next release (3.0.1). In the meantime you can use Google Chrome, Opera or apparently any other browser which doesn't use Firefox' JavaScript engine for using the Site Transfer Wizard.
The funny thing is that what seems to not be working is supposed to be supported in Firefox per the Mozilla Developer Network, Mozilla being the non-profit which develops Firefox. Well, browsers are definitely non-trivial pieces of software and things like that can and do happen.
Thank you for the heads up!
Nicholas K. Dionysopoulos
Lead Developer and Director
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