Do you perhaps have the quick filter toolbar enabled and have the unread button selected on it?

Quick filter toolbar was enabled but unread button was not selected. Selecting or unselecting unread button made no difference to showing missing emails Tried to remove or disable quick filter toolbar without successful

Check that you have View/Threads set to All, not Unread. Press Alt if the Menu Bar with View is hidden. The View setting seems to work independently of the Quick Filter bar.

Checked view settings was set to all but still cannot see old emails. Odd thing is that every now and then on opening Thunderbird all the email are visible, happens about 1 in 10 times. Also odd is that I use Mailstore to archive my emails and this somehow finds all of the hidden emails so at least I can recover at least part of critical ones that have been hidden by Thunderbird Frustrating to say the least. There must be some other setting that I have missed or has been corrupted.

Do you have the accounts set up as POP on other devices? The default setting for POP is to delete mail from the server after download, so other devices won't see them.