Frequently when I am composing several emails simultaneously, the TB display with the inbox disappears.

Please post a screenshot.
https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

This is the problem: there is no screenshot available to post. The emails I am composing are there, but the box that contains the inbox/trash/drafts/archives, etc. has disappeared.

the box that contains the inbox/trash/drafts/archives, etc. has disappeared.

I don't really understand what that means. What box are you talking about?
There is a main Thunderbird window (as opposed to a Write window) with a folder pane, and the folder pane has any of inbox/trash/drafts/archives, etc.

Yes, I mean the main Thunderbird window with the folder pane.

Are you sure the main windows is not getting minized to the system tray or taskbar?