Did you activate a filter in the Quick Filter toolbar, e.g. "Show only unread messages" ? (image) Do your Inbox messages reappear when you disable Quick Filter or if you restart Thunderbird in troubleshoot mode' ?

Thanks for the suggestion. The Quick Filter was on but the individual filter box choices were not. Turning Quick Filter off didn't reveal my old msgs or anything different. On left-side I see a box of folders with two sections (Recent Folders and All Folders) with an Inbox in both. Each Inbox line has two bubbles with different numbers in each. The Recent Folders' Inbox has the highest total number (300+); the All Folders' Inbox has 85. I can't open or access either Inbox. It is obvious I don't know how to manage these Inboxes. Nothing I click changes the listing of messages I just downloaded (47). BUT, it looks like my old msgs are still there, but somewhere I don't know how to access. That might be the key for me to access my older msgs? Side issue: Is there a user's manual for this TB?

Users often are "missing" emails because they are not familiar with the multiple ways to display messages in Thunderbird. You can read more about appearance, layout, message sorting and threading, ... etc in this help article https://support.mozilla.org/en-US/kb/appearance-panel-layout-message-list-card-view-tab#w_card-view

How many email accounts have you set up in Thunderbird? If you have several accounts the Inbox in "Recent folders" is not necessarily the same Inbox which you compare with in "All Folders". Which are your settings when you open the View menu and click on "Sort By"? (see image)