Stupid question regarding Thunderbird and IMAP

[Fred]

Hello, all.

I have a really stupid question that I hope some of you have an answer to.

I have a few clients that use Thunderbird as a IMAP email client to manage 
their email. They are having a dilly of a time creating subfolders within 
subfolders with Thunderbird -- and also Squirrelmail.

I am using Dovecot as the IMAP server. Is there some configuration detail to 
allow subfolder creation? Or are these email clients simply broken in this 
regard?

KMail has no problem in this regard, but my (human) clients are running 
Windows for the most part. :-(

Any suggestions? Am I missing something glaringly obvious? Or is there a real 
issue with Thunderbird (and Seamonkey and Mozilla and SquirrelMail)?

Thanks in advance.

-Freedom (and very busy) Fred