This happens in the dialog when attempting to enter a custom sync server URL. Soon after typing http:// or https:// or even if you type any single character and wait a few seconds, the words "Please enter a vlid server URL" appear under the entry field next to a X in a red box. After entering a complete and valid URL the above text never goes away and the "next" button never gets enabled. Including after filling all the rest of the dialog. that error message remains and it is impossible to go forward. This happens on all of my desktop Firefox installations. I can't test it on a mobile device as it is not possible to create an account that way. I've tried it with and without a port number in the URL.