I’ll assume you have installed the WebMail extension and at least one of the optional components, if you haven't please do so.
The first step is to check the WebMail extension is running. Go to the WebMail options, on the server panel the status of the three servers is displayed. The status for the enabled server(s) should be "Running". If the WebMail extension fails to start this could be due to the operating system blocking ports below 1024, try setting the port to value higher than 1024 and restart Thunderbird.
Check you have installed the necessary domain handlers. Go to WebMail options, on the domains panel is a list of supported domains.
POP Account Creation
From the main menu select File->New->Account.
In the Account wizard select “Email Account”
On the Identity page set your name and email address.
Enter the following server information.
Server Type : POP
Incoming Server : localhost
The user name MUST include the domain
Follow the new account wizard to the end
If you changed the port number during the WebMail setup stage you must change the port number for this account in "Account Settings" (tools menu).
SMTP Account Creation
Open “Account settings”, select the “Outgoing server” option from the tree and press the “Add Account” button.
Use the following server details.
Server Name : localhost
User Name : username@domain