Our editor is using the MOSS Asset Picker dialog to create links. This dialog has a known issue with the Firefox browser. Microsoft has fixed this issue in MOSS 2007 Service Pack 1. You need to either install the SP1 release or replace the asset picker dialog with the built-in RadEditor link dialog. To enable the internal link dialog, open the editor's tools file (\Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\184.108.40.206__1f131a624888eeed\RadControls\Editor\ToolsFile.xml) and rename the "MOSSLinkManager" tool to "LinkManager". Note that the built-in editor link dialog does not have the ability to browse the existing pages (no Browse... button). If you need this functionality the only available option is to update your MOSS installation to SP1.
the Telerik team