nickc that's strange. I don't use Firefox, and we've never had any error reports from other people.
After the form is submitted, the forum is created on the server-side, and then you should be redirected to the finish page. It might be just that this time the registration took longer to complete, or something else went wrong.
We'll have a look at this later. I have plans to change this behaviour to be API based and asynchronous with wider browser and OS support in the near future - mainly to prevent waiting after submitting the form and other rare errors like this one. Anyways, thank you for reporting this to us!
