popstate.dat is a file that Thunderbird uses to keep track of what messages it has downloaded from the server when using POP. These extra files are backups that are being created because of a problem.

1. Cleanup the Inbox by moving all the e-mails from it to other folders.

2. Find & open your Thunderbird Profile folder:

Select Help: Troubleshooting Information from the Thunderbird menu (either from the menu button , on the top right, or the normal menu at the top of the window.)

Under "Application Basics", beside "Profile Folder", click the "Show Profile" button.  This will open a window showing the contents of your Thunderbird Profile folder.

3. Quit Thunderbird, but keep that window open showing your Thunderbird Profile folder.

4. Delete "foldertree.json", "global-messages-db.sqlite" and "panacea.dat" from the profile folder.

5. Go to where these "popstate.dat" files are, and delete all of them, including the one without the number.

6. Start Thunderbird. It might take some time to download your e-mail and re-index all your folders. You might end up with some duplicate messages in the Inbox because they were downloaded already before.

Thank you, Bruce. Follow up question. Under step 4, "Delete "foldertree.json",.....", I have multiple "foldertree.json" files, with numbers after some of them. Do I delete them all?

darkphoenix said

Thank you, Bruce. Follow up question. Under step 4, "Delete "foldertree.json",.....", I have multiple "foldertree.json" files, with numbers after some of them. Do I delete them all?

Yes.

I suggest you create an exception in your anti virus program for the folders that contain your Thunderbird profile. Obviously something is interfering with the opening and writing of these files for multiple copies to be saved (the numbers are added because the orininal file is locked for writing. The most common reason files are locked like that is because the anti virus is scanning the files while they are being used and generally making a nuisance of itself.

Thank you, Matt.