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.

See https://support.mozilla.org/en-US/kb/glossary-terms-including-types-accounts

Is that what's happening?

Thanks for responding. I understand what you are saying but I've used this set up for many years and emails have always shown up on all devices. Both my iPhone and PC have read them and downloaded them in the past. Now my iPhone gets them but not the PC

Are you using IMAP or POP?

Check your Account Settings to see if you are leaving a copy of messages on the server.

am using POP. and yes copies are left on the server

I don't have a POP account with which I can experiment, so mark this as 'Not helpful', and someone else will help you. Good luck.