To explain further, I want to see all unified folders (Inbox, Sent, Trash etc) from various mail accounts grouped together as subfolders under one common Unified folder. Below it will be shown just regular account folders as in All Folders view.

Unified view in Thunderbird facilitates both POP and IMAP mail accounts. Creates a Unified virtual Inbox which contains all emails from all Inboxes from all accounts. It will also contain sub folders showing the contents of each accounts Inbox.

This is the same for all default folders. All created folders are then displayed below by account, followed finally by Local folders mail account. see image below as example.

Global Inbox: If all your mail accounts are POP mail accounts, then you could set them up as deferred mail accounts more commonly known as Global Inbox. The 'Local Folders' mail account is used for this. So the Unified account name is 'Local Folders' mail account. In this case, no matter what pop mail account, all incoming new mail will be displayed in the one 'Inbox', sent emails go into the one common 'Sent' folder etc. See info on POP mail accounts set up as 'Global Inbox'. This is not suitable for Imap mail accounts.

• http://kb.mozillazine.org/Global_Inbox

Toad-Hall said

Unified view in Thunderbird facilitates both POP and IMAP mail accounts.

What I mean is an add-on or feature that allows to create a new Unified Account, with unified folders from all mail accounts (Unified Inbox, Unified Sent, Unified Drafts, etc) grouped under it as subfolders.

Such Unified Account "All Messages" may be shown on the same view with All regular mail accounts, thus eliminating the need to switch between Unified and All views.

I think the answer to your question is we are not aware of an add-on. Perhaps you would be better served using Opera mail.

But perhaps you could look for an add-on yourself. https://addons.thunderbird.net/en-US/thunderbird/