When using FCKEditor 2.0 final I receive the following on ff when tryng to click on the "stuck down" b/i/u/sub/super icons:
Error: uncaught exception: [Exception... "Component returned failure code: 0xc1f30001 (NS_ERROR_NOT_INITIALIZED) [nsIDOMNSHTMLDocument.execCommand]" nsresult: "0xc1f30001 (NS_ERROR_NOT_INITIALIZED)" location: "JS frame :: http://localhost/path/to/js/fckeditor/e ... gecko_2.js :: anonymous :: line 13" data: no]
Works fine on IE. It does use a hidden div, but test case 4 is also failing so I think it is something more fundamental.
strong/em/u/sub/super are FCKd on firefox