I wonder what you think you mean when you say "Thunderbird account". You can add as many email accounts add you like to Thunderbird. I have no personal experience of iCloud but so long add they offer a standards-compliant email service via SMTP, and POP or IMAP then you should be able to work it via Thunderbird. In the traditional menu, go to '''File|New|Existing Mail Account''' to start the account setup process. If you're using the newer Application Menu button, it hides under '''File|New Message'''.