Have you ever had a prior account that was deleted? My awareness is that each new account bumps the number. Deleted account numbers are not reused.

Thanks for explaining, that make sense.

I’ve only ever had the same email accounts since I started using thunderbird, but 2 of them had to be setup again after strange issues started happening..

My iCloud inbox started to list every mail as coming from me and for some reason TB thought everything in the inbox was a template. I removed this account and set it up again.

My AOL account stopped recognising my password and all the mail disappeared. I had to setup a new AOL app specific password, then it redownloaded everything. This happened twice with this account.

So if TB counts each of the above instances as a new account rather being the same account, this explains why accounts 3, 4 and 6 are missing.

You're welcome. This becomes obvious if you browse the prefs.js file. All the account settings are there in simple text statement preceded with numeric account number. It's easier to create new numbers than to rename existing ones.