If I understand correctly, alias addresses rely on having a real address present. So, if you set up a real address, you could define filters that move mail sent to an alias to separate folders. And if you define identities for the aliases, you could set signature for each and compose, using the alias.

Yes, I can send and receive alias email with no problems. What I want to do is have Thunderbird show my alias 'accounts' as if they were real ones, in bold type and with their own set of subfolders, rather than having to have the alias 'accounts' as subfolders of the 'Local Folders' or some other account. Hope that clarifies things.

re :What I want to do is have Thunderbird show my alias 'accounts' as if they were real ones, in bold type and with their own set of subfolders,

Create a new existing mail account for the alias email address.