Thunderbird · IMAP · Help

Thunderbird 159: IMAP subfolders disappear

Check server contents, folder visibility and the documented IMAP compression regression before recreating folders.

Source date: 5 November 2026Editorial team: KAN KAI WORKSEditorially updated:

Quick answer

Missing IMAP folders in Thunderbird do not automatically mean their messages were deleted from the server. First inspect webmail, the exact Thunderbird version, folder views and subscriptions. An IMAP compression problem is documented for version 159. A matching temporary setting should follow a profile backup and comparison with the reported conditions; a visible folder list alone does not confirm complete message recovery.

Source basis: Q1 · Q2 · Q4

Applies to
Covers the documented Thunderbird 159 IMAP compression case. Subsequent fix information is version dependent; a planned correction does not prove a released or installed fix.
How to verify the result
Folders remain visible across multiple restarts and required contents match the server, while any temporary setting is documented with a clear route back to its previous value.
Your first check

Are the missing folders still present in webmail?

Compare the server contents and Thunderbird version first. Folders present on the server call for a different investigation from messages missing there too.

IMAP subfolders disappear after restarting Thunderbird, while the phone or webmail still shows them. A user report points to a specific Mozilla bug: Thunderbird 159 has a documented regression involving IMAP COMPRESS=DEFLATE. Q1 Q2 That does not explain every missing folder. Establish the actual server contents before choosing a repair.

Separate visibility from data loss

Check the provider directly. Folders also missing in webmail require a different investigation from a client-only display problem. Identify local folders and any data not yet synchronised separately. Back up the profile before changes; Mozilla's export instructions distinguish small and large profiles. Q4

Follow a specific sequence

  1. Record version and timing. Capture the exact Thunderbird version and whether the problem began after an update or only after a restart. The referenced bug concerns a particular version scenario, not every IMAP account.
  2. Check view and subscriptions. Inspect whether a view filter hides folders and whether the affected folders appear in the subscription dialogue. If they are present there, do not recreate them under slightly different names.
  3. Compare the documented symptoms. Review bug 2087454 against your observations. It tracks a fix for version 160. On the research date, that tracking status does not establish that a corrected build has been released on your channel or installed on your device. Q2
  4. Consider a bounded workaround. For the matching issue, developers document temporarily disabling IMAP compression with mail.server.default.use_compress_deflate=false. Preserve the previous value and a profile backup first. A knowledgeable administrator may use a narrower account-specific preference. Avoid unrelated changes during the comparison. Q2
  5. Compare after restarting. Observe the folder list across several restarts and compare messages with the server. A restored folder list is useful evidence, but does not replace checking locally stored contents as well.

Retire temporary changes deliberately

The Config Editor exposes advanced settings that can change between releases. Q3 Record the reason, original value and condition for undoing the workaround. After a verified applicable vendor correction, assess whether the temporary setting is still needed. A bug marked fixed is not, by itself, a complete update plan.

For handoff, provide version, provider, server/client comparison, subscription behaviour and timestamps. Avoid deleting the account or local mail files merely to force a folder-list rebuild. That can remove useful evidence and affect data unrelated to the display problem.

Frequently asked questions

Have my messages been deleted?

An empty or incomplete folder pane does not establish that. Check server and local contents separately.

Does this affect every Thunderbird version?

No. The documented regression is version-specific, and other causes remain possible.

Describe your case

Support with this issue

We can assess server contents, the client version and a limited compression comparison.

Describe your case →Related service: Data & improvements

Sources