I just discovered that the jSeblod CCK extension for Joomla has some layout issues with Firefox. Luckily there’s some posts in the Joomla forum with similar issues, although not necessarily related to jSeblod, but the issues are related to the display of HTML tables with the Purity template.
The fix as pointed out in the forum is to add a ‘float:left’ to the CSS for tables. Tracking this down to the exact table being using using Firebug, editing this css file:
templates/ja_purity/css/template.css
and adding this CSS fixes the issue:
table.admintable {
float:left;
}