Replace several accounts with one account
From MozillaZine Knowledge Base
This article describes how to use one account in Thunderbird to fetch and send mail for several accounts (usually 5- 10). Those other accounts have to exist on the mail servers, but don't exist in Thunderbird. This is not the same as a Global Inbox which hides the existence of accounts in the folder pane, the other accounts literally do not exist in Thunderbird. This simplifies how you manage your email in Thunderbird and outsources most of the work in checking for and fetching new mail.
It relies upon using multiple identities to change the From: address when sending, and having an account with a email provider that has a optional feature to periodically automatically fetch mail from a POP server and merge it into the webmail inbox. Your account fetches the mail for all of the accounts from that inbox. That feature is called Mail Fetcher in Gmail.com,  in GMX.com, adding an external POP server account in Fastmail.fm, adding an account in Yahoo! Mail Plus and adding a POP3 account in Windows Live Hotmail.
This article will use Gmail's Mail Fetcher. Unless somebody views the raw message source (you'd do that using View -> Message Source in Thunderbird) they can't tell that you sent messages from a Gmail account.
The main drawback of doing this (other than the work in setting it up) is that new mail from other accounts will not show up as quickly. Gmail's Mail Fetcher will check for new mail more frequently though in accounts that you get new mail in more often.
2. Sign up for a Gmail account if you don't already have one.
3. Create the Gmail account in Thunderbird. It can be either a POP or IMAP account. If you use a POP account you could configure it as a global inbox if you only want to see Local Folders in the folder pane.
4. Write down the email address and username/password of the other accounts. If necessary use Tools -> Options -> Security -> Passwords -> Saved Passwords
5. Move all of the folders in the other accounts to someplace safe, such as Local Folders. You might find it easiest to create a child folder for each account so that you can drag and drop the existing folders there without running into naming conflicts. Otherwise you need to select all of the messages in the folder and move it instead. You can organize your saved messages later on.
6. Compact the folders in each account, except for Gmail, by selecting the account in the folder pane and then File -> Compact Folders.
7. Delete the other accounts. You can delete an account by selecting it in Tools -> Account Settings, and then "Remove Account" from the "Account Actions" list box at the bottom.
8. Follow these instructions to tell Gmail's Mail Fetcher about the POP accounts you want it to fetch email from. If you want to replace an existing IMAP account, see if your email provider also provides a POP server. You might have to login to that email provider's webmail using a browser to enable POP access.
9. Add the email addresses of the deleted accounts as additional identities in the Gmail account. See Multiple identities per e-mail account for how to do that. Don't forget to register each email address at Settings -> Accounts -> "Add another email address" in Gmail webmail.
10. Your inbox will contain messages sent to multiple email addresses. If you want to make it easier to pick out who a message was sent to, use the Account Colors add-on to highlight mail using a separate color for each identity (email address). Another way to do that would be to use a message filter to add a colored tag based on what email address the message was sent to. Or you could create a separate folder for each email address (call it whatever you want) and use a message filter to automatically move new mail to it.
11. The multiple identity support is smart enough to default to the email address a message was sent to, when replying. When creating a new message, choose the email address you want to use from the From: drop down list. If you create separate folders to store mail sent to each email address you could use the Folder Account add-on to specify what identity (From: address) should be used when sending new mail from each folder instead.
12. Merge your saved messages from the deleted accounts.