Could you please clarify what you mean by an e-mail account ID#? Where in the user interface are you presented with the e-mail account ID#?

Via Config Editor With Thunderbird open, Click the Menu button (three horizontal lines) in the top right corner and select Settings. Scroll to the bottom right of the General panel. Click Config Editor... In the search box, type useremail. The list will show preferences like mail.account.account1.server or mail.identity.id1.useremail. The number following "account" or "id" is your account/identity ID#.

in my instance, I find two different instances of my gmail account, [edited]@gmail.com - one is id7, the other is id32. I assume only one is valid and contains my email folders, but I can't find any way to determine which id# is attached to the only set of folders for that account that exists in my TB frontend. And thanks for replying!

Open your Troubleshooting Information, and scroll down to the 'Mail and News Accounts' section. At the top right of the Thunderbird window, click the menu button ≡, then select Help > Troubleshooting Information.

Thanks, Chris, but I find nothing in Mail and News Accounts that helps. In fact, the account IDs listed there (account1, account2, account3...) don't even correspond to the id#s found via the method I outlined above.

I find two different instances of my gmail account, @gmail.com - one is id7, the other is id32.

An email account can have multiple identities. Basically this is an alias email address within an existing account. You can manage identities via the 'Manage Identities' button in your Account Settings. https://support.mozilla.org/kb/using-identities

I'd advice not to mess with 'About Config' settings unless you have a serious problem, and you know exactly what you're doing. So I'd suggest you just leave it alone.

Using the process you just shared, I checked to see if there were other identities provided in the drop-down when I go to send a message. I found all my other email addresses offered there, but there is only one instance of the gmail address in question. I assume that means I have never created another identity using that email address. I appreciate your advice about leaving Config Editor alone unless I really know exactly what I'm doing... obviously, I don't! ;) I wonder if this issue appeared as a result of having deleted the other gmail address that Google killed? The timing would be right.

We might be able to offer you more helpful advice if you could specify what you are trying to accomplish, or what problem you are trying to solve. I understand that it could be annoying or concerning to have an extra account ID# listed in the "back end" but it could be that it is not actually going to cause any problems, whereas, I would worry that it could potentially cause a problem to try to make any changes in the "back end" "Config Editor" AKA "Advanced Preferences".

Under the "Account Settings" for a given account, if I change the "Email Address" for that account, for example, if I change example@gmail.com to example@gmail.commeow, then I do find that this change is immediately reflected in the "useremail" list in the "Config Editor" AKA "Advanced Preferences". (Using two different proper names for the same user interface device -- brilliant as always, Mozilla)

So to reiterate, via the so-called front-end "Account Settings" for the account in question, you could temporarily change the e-mail address of the account, and see if that change then propagates to only one of the "duplicate" entries in the "useremail" list.

Even if you are able to determine that only one of the ID#s is correlated with your gmail account, I would still recommend against deleting anything in the "Config Editor" AKA "Advanced Preferences" that is associated with the other ID# that is not correlated. If you do make any such changes, I would recommend first backing up your entire mail folder, and, I would recommend being sure that you have a viable recovery process whereby you are able to recover from your backup, if needed. However, I would also recommend against messing around with backup copies of your mail folder, as this can get incredibly messy and confusing, and could potentially damage both mail folders, i.e. while testing out a recovery procedure you could end up downloading new e-mails to the backup copy of the mail folder, so then the original copy of the mail folder is missing those new e-mails... So I guess, any testing of backup and recovery procedures should be performed with mail folders and mail accounts that are used only for test purposes, such testing should not be performed with production folders and accounts...