Thread:Extension talk:Header Tabs/Doesn't work on MW 1.20.2 with IE, Firefox, and Safari

After upgrading a wiki from MW 1.19.2 to MW 1.20.2, HeaderTabs does not work in IE or Firefox. The title of the first tab is displayed as a bullet followed by the contents of the first tab. Subsequent tabs are not displayed. IE reports the following problem:

SCRIPT438: Object doesn't support property or method 'tabs' ext.headertabs.core.js, line 24 character 1

The line of code at that location is:

var $tabs = $("#headertabs").tabs;

Firefox does not report an error in Firebug but has the same erroneous behavior.