How do I combine Thunderbird setups each accessing different sets of multiple email accounts on 2 old computers into one Thunderbird setup on my new computer?

Question

The laptop accesses my personal email accounts (4 different ones) and my business email accounts (2 different ones). The old desktop accesses my personal email accounts and my wife's emails (2 accounts for her). I now have a new desktop computer and I would like to access all accounts from a single Thunderbird. It would be extremely tedious to start a new Thunderbird and enter all the account information for all of these multiple accounts from a fresh start and re-download all the emails. I know I can backup the Thunderbird profiles separately and restore them to separate Thunderbird instances on two separate users, but it is a pain to switch users each time one of us needs to see a different email account. If the multiple accounts cannot be restored into a singe new Thunderbird instance, is there a way to set up two Thunderbird instances on a single user's desktop and restore one computer's backup into one Thunderbird and the other into the second? Thank you.

Answer 1

Setting up the accounts anew in a fresh installation is probably the least tedious part of all this.
Merging your various email stores is ikely to involve moving gigabytes of data. You don't want to do that piecemeal.
I'd start with the most recent of your profiles and install that into the new Thunderbird. Then I'd add the required accounts whilst the old computers are accessible to double-check their settings for the accounts.
Then I'd import the mail stores from the older profiles using https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/
You haven't indicated if any or many of these accounts use IMAP, in which case all you need to do is set up the account on the new computer and the stored data will find its own way in.
While you have all your old profiles marshalled, it would be worth your while to import address book files too, using: https://nic-nac-project.org/~kaosmos/morecols-en.html to retrieve the *.mab files.