If this is for another email address.... 'Menu icon' > 'New Message' > 'Existing mail account'

So if I understand you correctly, you are saying this is how I establish a new account with Thunderbird? How does this guarantee that I will not loose what I have transferred over to Thunderbird?

I am worried about your repeated use of the phrase "new account with Thunderbird". What do you want from Thunderbird? Thunderbird does not provide email accounts. It's just an email client, used for managing email accounts and your email correspondence.

You can add any number of further email accounts to Thunderbird.

File|New|Existing Mail Account

Adding more accounts doesn't affect what's already there. Nothing will be lost.

The word you want is "lose". Take care not to lose your loose change. "Loose" rhymes with goose and noose.

Could you describe what you have done so far?

For example: you connected to an existing email account and got all the mail you wanted from the old account.

Did you use POP -- and put mail directly into local folders -- or use IMAP -- and put mail into a set of folders that mirrors what is on the server and then copy the mail into local folders?

Before disconnecting from your old account, you want to get all mail you care about into local folders.

For your new account, did you already open the account and get the provider's settings? If you're using one of the biggies (Microsoft, Yahoo, Google) Thunderbird may be able to connect automatically given your username and password. For other providers, it's a good idea to print out or save a copy of their server names and setup instructions.

This is what My Way had me do to switch over to Thunderbird: As a MyWay Email account holder, please log in and save all information you wish to retain. After October 2, 2017, you will not be able to access your emails.

We have enabled POP and IMAP on your account to allow you to download your mail onto your computer.

For added security, we have randomly generated a password for your POP and IMAP access. To access your email via POP or IMAP, install an email client such as Outlook, MacMail or Thunderbird and then configure them to contact to MyWay.com using the following settings:

Incoming POP server: pop.myway.com or Incoming IMAP server: imap.myway.com

Outgoing server: smtp.myway.com

I have transferred all of email messages, folders, and manually added my contacts to Thunderbird

Okay, great. Which did you use: POP or IMAP?

I just want to make sure you don't lose anything when you disconnect from MyWay, or mix together things that you want to keep separate.

I used the IMAP feature