Who is your email service provider? Is this an upgrade from v68 or older, or just an update for v78? Which antivirus program are you using?

Since you're using IMAP, your messages are on the server and you should be able to access them in TB 78.5.1 IF TB is able to connect to and communicate with the IMAP server. Missing MBOX files should not be a problem for IMAP accounts. Thunderbird will simply recreate them when re-downloading messages from the server if for some reason it couldn't use the existing file.

I have AVAST antivirus. And my ISP is called SFR (a French one). I do have my messages on webmail. I must admit that I don't know what the previous TB version was. I turned off the PC, restarted several time, but TB does NOT recreate the files. It is unable to (re-)download any message. I haven't tried to copy-paste the INBOX and SEND files that I had saved a couple of weeks ago, into C/. On my 2nd laptop, the upgrade didn't go as badly, since I still see the different files (incoming mail, sent, ...), but I can't receive or send any message. The box is like stuck in time, and I receive no error message. TB and the server no longer communicate.

The first thing I would do is remove Avast and use the built-in Windows Defender.

https://support.mozilla.org/en-US/questions/1312586

if your problem did not solve after disabling the AV/FW/SS (anti-virus/firewall/security-suite) software, then try an option from here (option-1: downgrade, or option-2: re-use old-profile with second thunderbird, or option-3: tranasfer specific files into new updated TB), etc).

Thanks for your suggestions. I disabled AVAST's automatic check on email and attached files, as well as the firewall, closed and opened TB again, finally turned off and started the computer. But it didn't make any difference. In TB settings, I downgraded security.tls.version.min from 3 to 1. Wouldn't work either. Thanks for the link. I'll see what I can try whether downgrading or fiddling with the .bak files wherever they are (quite hidden as far as I can see), though it seems that you need an IT degree to be able to perform the whole process. Thanks again.

I presume the imap mail account name folder is still in the left 'Folder Pane'.

• Right click on the imap mail account name folder and select 'Subscribe'
• Click on 'Refresh'
• Select all folders and click on 'Subscribe'
• click on OK

Restart Thunderbird.

Report back on results

Good idea. I hadn't tried that yet. The folder is still visible, but after clicking on refresh, there were no folders available. In the meantime I was told that the provider for the considered account had upgraded its security levels, which could be the reason why I can't see any files when I refresh the account. The simultaneous upgrade of BT and mail provider was quite an unlucky coincidence. I have asked for further information to the provider. I'll let you know if it helps solve the problem. Thanks.

Hi! The problem is solved at last. It was caused by new security protocols set up by the provider of the domain name following several attacks. There were also compatibility issues with the updated version of TB. The solution was eventually to change the IMAP port to 993, and to select SSL/TLS and the usual password. As to the SMTP server, the port was changed to 587 with STARTTLS and regular password. The SMTP address was also modified to smtpbyod.(domain name). It is confirmed that AVAST had nothing to do with it all. Thanks for your help.

My problem with this latest update is that THUNDERBIRD RESETS ALL MY CHOSEN OPTIONS, meaning, it now goes back to TB defaults. THAT SHOULD NEVER HAPPEN. I am a retired SE and this is totally unacceptable. ALL OPTIONS IN TB SHOULD STAY CURRENT TO WHAT THE USER HAS CHOSEN. Now, I have to continue returning to TB OPTIONS to change the defaults. IT IS SO ANNOYING and this happens every single update! STOP IT MOZILLA. JUST STOP IT.

I may move on to some other email tool if you cannot fix this unwarranted/unwanted change. And, not only that, this latest update 86.0b1 (32-bit) Beta has mashed my folder icons together, so they are hard to see because they overlap.

Thunderbird used to be a class act. NOW, it seems like grade school children are doing the programming on Thunderbird. I WILL NOT BE DONATING TO YOUR PROJECT ANYMORE IF THIS IS NOT RESOLVED.

STOP RETURNING MY CURRENT 'OPTIONS' FOR TB, TO THEIR DEFAULT VALUES!

I have attached a screenshot of my folder problem.

My problem with this latest update is that THUNDERBIRD RESETS ALL MY CHOSEN OPTIONS, meaning, it now goes back to TB defaults.

The original poster has already stated their problem is resolved, so your posting here is misplaced. Please state your issue in a new topic. And you have stated two issues, so you should make two postings. https://support.mozilla.org/en-US/questions/new/thunderbird/form

ALL OPTIONS IN TB SHOULD STAY CURRENT TO WHAT THE USER HAS CHOSEN.

Issue #1

this latest update 86.0b1 (32-bit) Beta has mashed my folder icons together, so they are hard to see because they overlap.

Issue #2.

But if you don't like breakage, then perhaps you should not be using beta?