Is this a POP account? If so, my guess is the popstate.dat file became corrupted, signaling to Thunderbird to download all mail. However, once done, the popstate.dat file should be ok. If the download repeats every time, it may be that Thunderbird is being prevented from updating that file.

I am not downloading to my computer. This is IMAP not POP. Hundreds of emails are being shown in Thunderbird that were written years ago and they are in bold as unread.. Why am I suddenly seeing purportedly unread Gmail’s from three and four and five years ago flooding my screen all of a sudden.?

Assuming the messages were deleted long ago, I suggest you contact your email provider. Thunderbird does not have the ability to download messages that don't exist. My suspicion from your post, is that your email provider restored old messages to the server and you are now receiving them.

Why are you making assumptions?. I don’t think I ever said they were deleted along time ago. They are just emails from years ago that are still in the Google server. They’re appearing in Tbird as if they are new. All of a sudden from years ago And as I have already said I am not down loading them to my computer this is IMAP not POP.

These are old emails from years ago that are in my Google storage folders are only now appearing on my screen in And they are in bold print indicating they have not been read yet. But they have been read years ago. They have never shown up in my Thunderbird screen view why are they here?

I will make an assumption, and you are not going to like it.

The read status of email is synced in an IMAP situation so we are seeing here one of two things. A complete failure of Thunderbird to link the read status of mails over a period of years to your providers web site. As I like many millions of others use Thunderbird with Gmail in an IMAP situation, I can confirm that while it can happen due to timeouts in connections it is rare, and only occurs for a short period. Like you mark a folder read and only half of them are marked before the connection ends for some reason like the power it off.

The second is you have something on your system that is causing issues (Not Thunderbird) although it could be an addon). So when you access your email on the gmail web site using your browser are these email marked as read as they should be or not? That is the first step is trying to determine what is happening.

All of this is speculation and none of it is the least bit helpful. Thanks but I already have a career and don't need another one running down guesstimate rabbit holes to mange a simple email account. I am also running Outlook (which I want to dump for other reasons and that's why I am trying Tbird) but the problems I am reporting here are not duplicated at Outlook or Gmail so clearly reside with Tbird. And if support at Mozilla can not understand and resolve email issues that reside only with the use of it's product then this 'free' resource it's way too expensive to use because I do not have time or interest in speculative guesses offering no solution. So I'll look for a different product with better support. In any event unless you think you understand the problem and can offer a concrete solution based on verifiable presumptions then please leave this question to others to answer. Thanks