For security reasons Firefox limits the permission to paste content using javascript, but you can configure the program to grant that permission in a domain basis.
If you doesn't like to configure your system, use [Ctrl]+[v], that will work as a last resort. But if you like follow the next steeps.
1. Close all Firefox windows/tabs.
2. Create a text file named user.js in your PROFILE folder (see below). On Windows use notepad (make sure you can see file extensions first so you don't create a file named user.js.txt).
On Windows XP your PROFILE folder is usually on:
C:\Documents and Settings\[UserName]\Application Data 
\Mozilla\Firefox\Profiles\Default.Oj
according with your system language and similar for Windows 2000.
Do NOT use the Firefox/Mozilla program under
C:\Archivos de Programa\Mozilla\
On Linux/Unix your PROFILE folder is on:
/home/[username]/.mozilla/firefox/[someletters].[profile]/
3. Add the following lines to the user.js file (replace 'yourdomain' with your real domain where AldoContent is used for):
user_pref("capability.policy.policynames", "allowclipboard");;
user_pref("capability.policy.allowclipboard.sites","http://www.sudominio.com");
user_pref("capability.policy.allowclipboard.Clipboard.cutcopy","allAccess");
user_pref("capability.policy.allowclipboard.Clipboard.paste","allAccess")
5. On Windows save the file, close notepad and restart your computer.
On Unix-Like systems you don't have to restart as usual 
.
Now you should be able to cut-and-paste in FCK editor.
