Same message here.
As far as I can see, it only works on Inbox (to mark as Spam) and McAfee Anti-Spam (to mark as not Spam) folders.
I assumed it was an (absurd) feature.
I get the message error message "You cannot perform this action in the current Thunderbird folder" on my local Inbox and online Inbox folder. McAfee has not created a anti-spam folder in Thunderbird, which according to the online help files it should have. I have re-installed Thunderbird and McAfee(used MCPR tool)Message was edited by: coxy on 1/25/11 2:16:19 PM CST
Just to recap our discussion earlier, Anti-Spam plugin does not work with IMAP account at the moment. If any users are experiencing the same problem as coxy, you need to reconfigure the account type to POP3.