You mention 'Mozilla or Comcast or Microsoft' - For 'Mozilla' I presume you mean 'Thunderbird' For 'comcast', I presume you mean that is your mail account/email address. For 'Microsoft' I'm not sure whether you are talking about your 'Windows 11 OS' computer or whether you also have a Microsoft account and email addess eg: outlook.com.

It sounds like you have more than one computer accessing same mail account/email address server.

On all of your computers, phones etc are you connecting to the 'comcast' email address via an Imap account? Or have you got a Pop account on one computer and an Imap account on another ?

You mention 'Windows 11', so how did you get the download for Thunderbird - was it downloaded from the official website : https://www.thunderbird.net/en-US/thunderbird/all/ OR did you install it via the 'Windows app Store' ?

Please list all devices and tell us what version is installed on each of your devices ?

(1) Mozilla means Thunderbird, Comcast is email provider, Microsoft is Windows 11 not email provider.

(2) MacBook Pro Ventura 13.7.5, IMAP, Thunderbird 128.10.2esr(64-bit)

     HP Elitebook 840 Windows 11 Pro 24H2 Build 26100.4061, Thunderbird 115.14.0
     MacBook Air Monterey 12.7.6, IMAP,  Thunderbird 128.10.2esr(64-bit)
     Dell Inspiron 5593 Windows 10 Home build 19045.5854, IMAP, Thunderbird 115.18.0(32-bit)

(3) I downloaded Thunderbird to the Windows 11 probably from Mozilla but don't actually recall. It has worked just fine for nearly 1 year.

(4) at least today, it's the Windows 11 laptop with the empty folders, but the filter and disappearing folder problem seem to be happening randomly, on both MacBooks and Windows.

thank you for answering.

So you have two MACs using IMAP and current up to date versions 128.10.2esr AND two different Windows OS, both using IMAP, but also using older out of date versions.

Is there a reson why you have not got both Windows 10 and Windows 11 using 128.10.2esr ? If you would like to reinstall, then you can get it from:

• https://www.thunderbird.net/en-US/thunderbird/all/

BUT please take care when selecting - Channel Release to select: 'Thunderbird Extended Support Release' option as that will get the 128.10.2 esr version.

IMAP accounts mean all emails are stored on server - the view you can see if you logon to the webmail account via a browser. All folders you create in webmail account or on any computer imap account will be stored on the server. That means all imap accounts on all computers can access the server to see anything on the server.

IMAP accounts subscribe to see the folders on the server, so if you have a folder called 'Bills', it will be seen if you logon via webmail OR via any of your four computers providing you 'Subscribe' to see it. That means you do not need to create a folder that is already on the server, you just need to subscribe to see it. If you can see a specific folder using eg: Windows 10 or Macbook and it's also visible in webmail, then you can Subscribe to see it in Windows 11 computer.

It also means if you delete or move a folder to be a subfolder etc in eg: Macbook computer then it's going to be either deleted or moved on the server. So when another computer eg: Windows 11 is started up, the IMAP account is not up to date. It's log of folders is now out of date with reality. It will have info that a folder in a specific position no longer exists in that position meaning you will need to subscribe again to see that folder.

How to subscribe to see a folder that already exists on server:

• In Folder Pane
• Right click on imap account name folder and select 'Subscribe'
• Click on 'Refresh'
• Select the folder you want to see and click on 'subscribe'
• Click on 'OK'

re :1) Over the past year (maybe more?) folders move in Thunderbird seemingly by themselves.

Folders are always in a specific order. At the top will be the default server folders. Names may slightly vary eg: 'Sent' or 'Sent Mail' or 'Sent Items' - it just depends upon the server. Inbox, Drafts, Templates, Sent Mail, Archives, Spam, Trash They will be followed by a list of the folders you have created and they will be listed in Alphabetical Order.

The View you see depends upon what you have selected. By default 'View' > 'Folders' > 'All' is selected. Some people use 'View' > 'Folders' > 'Unified', but in your case that's not useful because you only have the one comcast mail account. Please confirm you have 'View' > 'Folders' > 'All' selected on all computers.

When you say a folder moved - can you offer more info - moved from and to where ? It could only get moved either from a top level folder to being a subfolder or vis versa within the same imap account. If you moved it on any other computer or even via webmail then it will show as moved on all computers. So naturally, any filter that needs to move an email to that folder will no longer work.

re :(2) Occasionally folders disappear completely, so I have to recreate and repopulate them. If a folder disappears from a Windows 11 computer - does the same folder disappear from all computers at the same time and also - this is important - disappear off the server as seen if you logon to webmail account via a browser?

Each computer will store it's own profile and computer puts it in a specific location, but that location varies depending upon OS and where you got the Thunderbird download/installation.

re :(4) at least today, it's the Windows 11 laptop with the empty folders,

Empty folders can be due to several settings. The first most worrying is whether you have an Anti-Virus that is allowed to scan your Profile folders. Windows 11 comes with a default Anti-Virus or maybe you have installed one. Advise you do not allow any Anti-Virus product to scan any profile folder for file because it cannot tell if one mbox file is just one email or thousand - it can end up deleting the contents of an entire mbox file and that means an entire folder in Folder Pane. You need to make your Thunderbird profile exempt from scanning. If you have an folder lean up software - please make your Thunderbird profile exempt from scanning as you do not want any session data etc being deleted.

Once that is done we can rule out other programs interfering.

But to do it - it means you need to know where is the profile located. If you installed Thunderbird from the official Thunderbird download page then this will be the location of your Thunderbird profile folder:

• C://Users/username/Appdata/Roaming/Thunderbird

If you installed via a 'Windows App Store' then it will be here:

• C:\\Users\username\AppData\Local\packages\MozillaThunderbird.MZLA_h5892qc0xkpca\LocalCache\Roaming\Thunderbird

Please check both locations and tell me what you discover. Have you got it in both locations or just one and which one?

If all the other computers and webmail account still have those same folders containing emails, then it might be just a view issue or Folder index needs remapping. TRy :

• Right click on folder and select 'Properties'
• click on 'Repair folder'
• click on 'OK'

THANK YOU for such a thorough answer. I'll let you know how things stand when I've checked everything, maybe installed the updated version on Windows, etc.