When removing an account, the pane that pops up has a tick box to specify whether to delete message data as well as account data. That lets you preserve messages when account is deleted.

David, replies other similar past questions seem to indicate that deleting an account = deleting all emails in local folders, hence my question. See for example: https://support.mozilla.org/en-US/questions/1159453. That thread pertains to a gmail account, which I guess is IMAP and a different story than deleting one of several POP accounts on Thunderbird? Would appreciate if you can say more about this as I have 10 years of essential work emails stored in the Thunderbird accounts I plan to delete.

How can I backup these emails before deleting the accounts?

Thanks again!

That other problem was IMAP. For POP, this is the pane that pops up when clicking 'remove account.' Notice the tick box. Leaving unticked will leave the message files, but will NOT be visible within Thunderbird (but are there to see in windows file manager.) My suggestion, since you want to be fully safe, is this: - install importexporttools-NG if not already done. - in Local Folder, create a new folder with name of account (or whatever you wish). for my example, I will call it XXX - now, highlight one of the folders in account to be deleted. Press cntl, hold down while dragging folder to the XXX folder - repeat for all other folders. - now, pause and review that all is well. If you're a bit paranoid, exit thunderbird and restart just for sanity. - at this time, you should have all POP message folders in Local folders, and can now delete the account. You can now tick the box to remove all account data. - final step is to do backup: - exit thunderbird (IMPORTANT) - using windows file explorer, copy the full profile to external media, possibly to two separate media if messages are vital. - if you're unsure where profile is, click help>troubleshootinginformation, scroll down left side to 'profile folder' , click 'open folder', and then EXIT thunderbird, you are now in the profile. click the back arrow two folders so that you're at the folder named 'Thunderbird.' That's the one to copy.