Hi,
we have found the problem:
in firefox password field is partially filled (see screenshot), this generate a error because password and confirm password do not match.
Why you not see any error message? I think that in your template missing code to show Joomla messages, you need to add in your template (index.php) this code:
<jdoc:include type="message" />
see Joomla docs at:
https://docs.joomla.org/Display_error_messages_and_notices