I'm running Thunderbird 156.0 on Windows 11 with ten IMAP accounts (Gmail, Outlook.com and Zoho) plus one Exchange account served by the Owl add-on, and until recently ab… (read more)
I'm running Thunderbird 156.0 on Windows 11 with ten IMAP accounts (Gmail, Outlook.com and Zoho) plus one Exchange account served by the Owl add-on, and until recently about 30 GB of mail held locally. On starting Thunderbird, folders are destroyed and re-created empty. It happens often, but not every time. The summary files are deleted, and in some cases the whole folder directory goes with its mbox message store inside it. The folders then come back in the folder pane holding nothing, and Thunderbird downloads all the mail again over several hours, re-indexing as it goes. When a folder directory has been re-created, it appears on disk with a numeric suffix, so one of my folders now lives at COMPLIANCE-1.sbd with an abandoned COMPLIANCE-2.sbd sitting beside it. Column layouts and favourite-folder flags are lost at the same time, because they live in the summary files. It happens on ordinary restarts, rather than after Windows updates or sleep.
Those orphaned directories have let me date three separate occurrences: 18 August 2025, 19 February 2026 and 19 September 2026 — those being the occasions that happened to leave orphaned directories behind. Most leave no trace, because the folder refills and the evidence is overwritten. Each has the same signature. A folder directory is created, and roughly five minutes later a summary file of about 1,200 to 1,500 bytes, which is an empty summary, is written into it. In the two older cases it was never touched again. In one event eleven summary files across two folder trees were written with an identical timestamp to the second, so it appears to be a single operation across a set of folders rather than something that walks through them one at a time. One of the destroyed folders left its mbox stores behind, orphaned with no summary file, while another was deleted whole with 50 MB of mail inside it, so there seem to be two different outcomes in the same event. Bitdefender is running, but its quarantine holds nothing from the September event, and there are no Thunderbird entries in the Windows event log for that period.
The part that may be useful is that I have reproduced it. On 19 September I renamed folderCache.json aside with Thunderbird closed and restarted. Eight minutes later folders were destroyed. The twelve I could identify were all children of INBOX, across two different mail providers, though other folders elsewhere were emptied in the same minute. The Exchange account served by the Owl add-on was untouched. At the moment of destruction the Activity Manager showed "[NONEXISTENT] Invalid folder: INBOX^Practice Filing (Failure)" on the Gmail account, where the correct hierarchy delimiter is a forward slash rather than a caret. My folderCache.json is 5.2 MB with about 13,000 entries, of which roughly 1,200 point at summary files that no longer exist and 9,600 sit under an obsolete lowercase profile path from the 129 to 139 version window. I would rather not delete it again given what happened, so I'm looking for any idea of what triggers this on an ordinary restart, and whether there is anything I can set that would stop Thunderbird discarding local folders when it decides they are missing.
I am going in circles with this. If I build a new profile, is there any point bringing the old configuration across, or would that just reimport whatever is wrong? I assume the address books and filter files are safe and that folderCache.json, prefs.js and the unified-folders directories are not, but I do not know whether the problem lives in the profile at all, or in how Thunderbird handles an account set this size.
Any ideas?
