You get mx records from your provider. Thunderbird is email client software, not a provider. There is no such thing as a Thunderbird account.

MX records are public information because mail servers use them to route mail to your domain.

To look up the MX records for your domain, you can use this query tool:

http://www.kloth.net/services/dig.php

Change the query type from A to MX.

Now... Why is Network Solutions asking for this information?

• If they were planning to take over hosting your mailbox, then they would be creating new MX records pointing to themselves. Your old MX records would be irrelevant. Your old mailbox will stop receiving new messages once mail servers globally have expired all cached references to your old MX records.

• It sounds more like they just want to create correct DNS records for your domain so mail can continue to flow normally to your old mail service provider. Better check on exactly what's happening before you pull that plug so you don't lose both your old mailbox and all inbound messages by terminating that service prematurely.

If you need to manually move your mail from old-server to new-server, there might be a Knowledge Base article on how to set up two IMAP accounts simultaneously and copy mail between them, or to make a copy into local folders before disconnecting from the old server. Do not disconnect Thunderbird from your old mail account without making some kind of copy of your mail outside of that account's folders.