There's a security setting in Firefox that prevents javascript-based HTML editors from accessing your clipboard.
There's a workaround using a Firefox add-on called AllowClipboard Helper
https://addons.mozilla.org/en-us/firefox/addon/allowclipboard-helper/
Some documentation written by Ottawa University:
http://onl.ottawa.edu/documentation/student/docs/How-to-fix-copypaste-issue-content-editor.pdf
MozillaZine:
http://kb.mozillazine.org/Granting_JavaScript_access_to_the_clipboard