Make sure Global Inbox is turned on for all POP accounts. https://support.mozilla.org/en-US/kb/unify-your-pop-email-accounts-global-inbox

The reply from christ1 is not addressing this problem. That reply is applicable to POP accounts, and downloading the messages into a local folder referred to as a Global Inbox. That is a totally different concept from the unified inbox problem that has just occurred.

To clarify the problem... All my email accounts are IMAP I use the menu option View | Folders | Unified With this turned on, the folder list begins with "Unified folders". Under this is "Inbox", and if you expand it, under this are all the individual inboxes.

Until recently, messages in all of the individual inboxes were unified and shown in the topmost folder "Inbox". Now, the messages from SOME of my accounts are missing from the unified "Inbox" and are only visible in the individual inboxes below it. Other accounts are still working fine and are appearing in the unified "Inbox" as well as the individual inboxes below it.

BillSparrow,

please start a new topic for your problem. https://support.mozilla.org/questions/new

christ1,

My problem is identical to the original poster's problem. I was merely adding a more detailed explanation of the issue. On the other hand your reply referred to something else altogether.

The solution to this problem is posted in the replies to "One mailbox is ignored in the unified folders"

I will attempt to post a link to that question. Hopefully it will work... https://support.mozilla.org/en-US/questions/1392443

This is an older way to show inboxes as "unified" and only works with POP (not IMAP) servers. The "modern" way is to stack the multiple accounts using the Thunderbird View menu, then selecting "Unified" view.