There are several possible reasons that messages seem to 'disappear'.

There are two types of email accounts: IMAP and POP. See https://support.mozilla.org/en-US/kb/glossary-terms-including-types-accounts

If you have a POP account and are accessing the mail on more than one computer or smartphone, whichever device reads (downloads) the message first will have it locally, but it will no longer be listed or available on the other devices.

If you have an IMAP account, the message stays on the server, but if one device (user) moves or deletes the message, it will affect the view on all devices. So, for example, if you are using your smartphone and take some action on an email, when you get back to your computer, it will reflect that same action (but you might not have realized what caused it).

Sometimes the index of a mail folder (.msf) will get corrupted. If you right click on the name of the folder and select Properties, a 'Folder Properties' window will open. Select 'Repair Folder'. This may take care of the problem.

Another, less likely, reason for messages not to show up in your inbox is having the preference set to show only unread or recent messages. Then once you read them they seem to disappear.

If you are still having problems, please look at the following items and respond with the information.

   1) What operating system do you use (Win8, MAC, Linux, etc)?
   
   2) Are you using IMAP or POP?

   3) If your mail server has a browser interface (web mail access such as mail.google.com), do the messages show up there?
       
   4) Right click on the folder in question and select Properties.
       a) Number of messages?
       b) Size on disk?
       
   5) Left click on the account name; View settings for this account; highlight Server settings;
       What choices are marked under "When I delete a message"?
   
   6) What, if any, antivirus program do you have installed?

Hi- thanks very much for replying. I have tried the "repair folder" a few times, with no result. The account is AOL, and when I logged in directly the emails are gone from my "old" (read) mail box, as well. Same on my other computer and phone. I am using Windows 7, and I am using IMAP. The Size on disk is 62kb. For the option " When I delete a message:" It directs it to move it to the trash. For the record, the trash folder and all other folders with this profile and 2 others are intact, and the trash in this profile is what I have specifically deleted, not the emails I left in the "read" folder. I have Norton 360, and have used it on this computer for years. I am thinkin now that AOL has something to do with this, but can't find any answer there, either.

did you by any chance delete the read messages from your phone? IMAP Synchronisation is a killer for the unsuspecting.

No, I did not. I usually delete from device only. The day it happened I hadn't even looked at emails on my phone. It was intact in the AM and then gone suddenly early afternoon. Would the phone do it without a prompt?

Thanks for all the assistance! As I have been digging deeper into my folders, there are a ton of emails in the trash folder. I am sorting and saving the ones I may need, but there are many that I never deleted from my mail, so this must be the place. It's nice to have a chance to make sure valuable info is available, but forced cleaning is a small gift too....

-)