I was unable to locate those files on my PC so I assume they don't exist with my version of Thunderbird.

They have existed in every functional copy of Thunderbird ever released. POPstate.dat for all pop accounts and MSF files for every list shown in the main user interface. Inbox.msf being the list shown in the inbox folder. Your inability to locate them not withstanding at least one of them will exist, probably both. Go to the help menu select more troubleshooting information and then the open button for the profile folder.

Windows search (which most folk making you statement use) will not find any of the files shown there, or any in sub folders other than some wdseml files if you have enabled windows search in Thunderbird. I have not. In my case that is some 50Gb of files. Microsoft do offer setting to make these visible, they just are not enabled by default.

Constant re-downloads is a known issue in some IMAP accounts, but mainly listed for accounts with thousands of folders. But you are talking about a popstate.dat file. This is for pop accounts. Is you account a POP account? If so the following might apply.

There is also a known issue with pop account and some anti virus products. Norton is most frequently mentioned in that context but failure to perform is not limited to that software. The anti virus just drops the ball when it encounters an email that is not formatted how it likes so the connection just times out while the AV goes off to la la land. In a few minutes when the download of mail starts again the same emails are downloaded again until it get to the malformed one and again the anti virus drops dead and it times out again. Does this sound familiar. If so you can fix it by turning off incoming mail scanning. Otherwise use web mail to empty the spam folder (most malformed mail is spam) and then start deleting mails that arrived immediately after the ones that have downloaded until you strike the one that is causing the issue and the download restarts stop occurring.

I do not have mail scanning enabled, so I never see such an issue on POP accounts.

Sorry for any confusion. The account in question is an IMAP account. It contains less than 10 folders. Any solutions/mitigation for constant re-downloading on IMAP accounts? Thanks again!

With what provider?

If you're asking for the 'server name', it's imap.gmail.com

There is 10 years of discussion on Gmail and this issue here. https://bugzilla.mozilla.org/show_bug.cgi?id=816327 The last one I read there suggested right clicking, selecting properties and then the repair button.

Thanks Matt. Time will tell if repairing folders does the trick. Thanks again!