Filter rules are stored in a msgFilterRules.dat file for each email account inside the Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data

You can copy those file(s) from the old machine, and paste them to the corresponding location on the new machine. Do that while Thunderbird is closed.

Hello there

Locate the msgfilterRules.dat file Locate the msgFilterRules.dat file on the old computer.Windows Location: %appdata%\Thunderbird\Profiles\xxxxxxxx.default\ImapMail\imap.<account>\

Steps:Copy the msgFilterRules.dat file to a USB drive or cloud storage.Paste the file into the corresponding account folder on the new computer while Thunderbird is closed.Restart Thunderbird on the computer machine.

Alternative Method Open Thunderbird, go to Help > Troubleshooting Information.Under the "Profile Directory" row, click Open Folder (or Show in Finder/Folder).Browse into the Mail (for POP) or ImapMail (for IMAP) folder to find the file.Migration Steps:Copy the msgFilterRules.dat file to a USB drive or cloud storage.Paste the file into the corresponding account folder on the new computer while Thunderbird is closed.Restart Thunderbird on the computer machine.

I copied the msgFilterRules.dat from the Mail\mail.friesen.net subdirectory to the Mail\Local Folders subdirectory, as there is no equivalent to mail.friesen.net in the new AppData, just a LocalFolders. It did not work. The filters are still empty.

I had the old installation setup to download the Inbox and remove the messages from the remote server. The new one does not seem to be configured that way.

Hello there

Can I ask you to copy these files again while Thunderbird is not running at all? If Thunderbird is running when you make the copies, the files can become corrupted. The files were copied while Thunderbird was still running on one of these PCs.

The msgFilterRules.dat file has become corrupted or changed with valid/invalid characters/symbols. Because Thunderbird cannot read these, it ignores the file, leading to disappearing filters.

Use Please try using this add-on to resolve this issue. https://addons.thunderbird.net/en-us/thunderbird/addon/importexporttools-ng/versions/?page=1#version-14.1.18

Tip Try to delete the newer version on the new PC and reinstall it. It might be that you are using a changed or otherwise installed Thunderbird installation.

I had the old installation setup to download the Inbox and remove the messages from the remote server.

Not that I really understand your filter setup, but you may need to exactly replicate the account setup, and folder hierarchy from the old computer on the new computer.