Is your issue with Mozilla Thunderbird or are you referring to an email website that you are having issues with?

If the issue is with Thunderbird, it sounds like you may have activated the Quick Filter Toolbar by accident. This can allow you to display on unread messages. If that's not it, you may even be using a Saved Search, which can have the same effect.

If you have this issue when viewing a specific email website in Firefox, let us know which one. There is likely a filter activated there.

Thank you for your reply Wesley. The problem is with Thunderbird, not any other website. This does not solve my problem. I think I may have clicked unread in the tool bar by accident. But seems viewing only unread messages should revert to showing read and unread as usual. I did it after receiving your message and then I had to search for it to answer it. It is shown as being in the inbox but doesn't display in the inbox.

I have disappearing files on my devices other than my original computer, where I set up my thunderbird account, since I bought the original computer I have purchased an iphone and a laptop, both receive emails but when I open the original computer they disappear, the emails are still on the original computer

Email providers will usually offer two connection types: POP3 and IMAP.

If you connect using POP3, the email will be downloaded by whatever email program loads the email first and then it will be removed from the email server. For example, if you check your email on Computer A first and then Computer B, any emails Computer A loaded won't appear on Computer B.

If you connect using IMAP, the email will be downloaded by any email program, but will be left on the email server. This allows multiple devices to see the email.

See The difference between IMAP and POP3 for a more detailed explanation.

It sounds like you are probably connecting via POP3, which is not ideal when you are using multiple devices.