is you account IMAP or POP. I would guess IMAP and you are not aware your providers retention policy applies to the local IMAP account. But that is a guess.

Hmmm. Nope. Don't know. It didn't seem to matter using Win7 on my old computer. I had hundreds in the Inbox at one point and didn't lose any. Will check this out though. Thanks for your reply.

Depending on how you "moved" to your new computer you old may have been a pop account and the new an IMAP one. Folk think of email as this homogeneous blob but it is actually a collection of protocols and standards and your choices when setting the program up on your computer make huge differences.

Thanks Matt. Sorry it took so long to reply. Holidays, ya know. Thunderbird reaches out and sets up the mail program automatically so it picked up IMAP from my provider. And. I checked some settings on my providers mail box and found a setting for how long to keep the emails. I never saw or used it before so didn't know it was there. I changed from keeping email for 2 weeks, to, unlimited. So that should solve my problem. That is why old emails were dropping off/disappearing in the Thunderbird Inbox. I appreciate your help. Happy New Year!

The change in my providers settings from keeping email for 2 weeks to unlimited solved the issue. The emails are no longer disappearing from my Inbox in Thunderbird. Thank you for your replies.