Problem with jquery dialog width in Firefox

Problem with jquery dialog width in Firefox


I'm having a problem with the "width" option of the jquery ui dialog
widget.
I'm using jquery 1.3.2, and jquery ui 1.7.1.
I'm opening a dialog with the following code:-
$("#popup").dialog({ height: '400', width: '800' });
In IE7 (7.0.5730.11) this is displaying correctly, however in Firefox
(3.0.8) the "width" CSS attribute of the dialog is not being set. See
below for an example:
http://www.iaingalloway.com/DialogTest.htm
I expect I'm doing something obvious wrong. Could someone give me a
hand?