Signup page display problems in Mandigo

I've been having a terrible time trying to get my signup page to display properly using the Mandigo theme (v. 1.29 in the 100 big ones theme pack).

Check it out ...
http://rvblogz.com/wp-signup.php

The page looks great in Safari, Chrome and IE7 but gets chewed up in Firefox. The form fields do not look as pretty, and the "I Agree" label for the TOS plugin is crammed next to the textarea with the check-box on the next line.

In IE6 the fields look OK but everything is crammed into the left 20% of the browser frame.

In all browsers, the signup security question plugin displays below the next button.

I'm using the following plugins, but deactivating any/all of them makes no difference to the layout.

  • signup_question.php
  • signup-tos.php
  • signup-password.php

Any advice on how I might troubleshoot this is greatly appreciated. Of course any suggested fixes without editing core files would be a blessing.

Thanks to all in advance for any suggestions!