I've got k2 v2.4.1 running.

 

I've got 4 extra fields setup as Textareas with 'Use Editor' selected.

 

The editor appears correctly in  Chrome , Firefox (3.6.14) and IE8, but is only editable in Chrome and IE. Firefox generates the following error when I highlight some text and click on any menu item.

 

Error: uncaught exception: [Exception... "Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMNSHTMLDocument.execCommand]"  nsresult: "0x80004005 (NS_ERROR_FAILURE)"  location: "JS frame :: WEBSITE/administrator/components/com_k2/lib/nicEdit.js :: anonymous :: line 38"  data: no]