When you click a page or a link that opens a dialog in the author environment using Firefox 3.x, two windows open.
The problem is one JavaScript line inside the List.esp.
As a quick workaround, comment the line 1374 in /libs/CFC/templates/Admin/List.esp (or if you overlayed look in /apps/CFC/templates/Admin/List.esp)
...... // --- workaround for firefox behavior --- if (!ie4) location.href=myLinks[fini].link.href;