Checked 9 September 2026. Thunderbird 155.0 was released on 1 September and fixes automatic IMAP tag synchronization, among other issues. An update is not ready for everyday work until messages, local folders and essential extensions work. This guide is for home users and small offices using Thunderbird Desktop. It is a source-based checklist, not our own installation test or a guarantee against data loss.
Preparation: what you need
Allow time without urgent outgoing mail and enough space for a complete profile copy. Record the operating system, installed version, installation source, extensions and accounts, but not passwords. On a managed work computer, let IT control the update. Release and ESR are different update channels; a lower ESR version number does not automatically mean an update is missing. Do not switch channels simply to obtain the number 155.
Step by step: backup and update
- Record the baseline: open a familiar older message with an attachment and list the local folders, address books and calendars you actually need. This helps identify pre-existing omissions later. Record the version from Troubleshooting Information; opening About Thunderbird may already trigger an update check.
- Locate the profile: open Help, then Troubleshooting Information, and open the profile folder. Quit Thunderbird completely. Copy the entire profile folder to a separate protected backup location; do not move it or overwrite an older copy. Back up every active profile. Include any message stores configured outside the profile.
- Check the copy: confirm that copying completed without errors and that the folder size and important subfolders look plausible. Label the backup with the date and version. It may contain messages and credentials: never upload it to public shares or support tickets. IMAP synchronization alone does not replace this backup.
- Use the appropriate update route: for a directly installed copy, open Help → About Thunderbird; on macOS, About Thunderbird is in the Thunderbird menu. Download the offered update and restart when requested. For Linux packages, Snap, Flatpak or centrally managed installations, use the existing package or management route; do not install a second copy over it.
- Check version and channel again: the goal is the current version offered for your channel, not forcing exactly 155.0. A later maintenance release may be newer. If the old version remains, check the installation source and error before reinstalling. Do not disable security software or certificate verification.
- Accept the workflow: perform the checks below for each required account. Keep the backup even if the first launch succeeds. A successful application update does not prove that restoration has been tested.
Verify the workflow, not just the version
Open the previously selected older message and its attachment, check local folders and a known contact. Send a harmless test message to a second address you own and reply from there; check the inbox and sent folder. For IMAP, compare a tag on a test message between two clients that support it, without deleting production mail. If used, check calendar display and a typical extension function. A separate restoration trial belongs in an isolated environment without automatic account connections, to prevent old data synchronizing with the server. Backup size and existence alone do not prove recoverability.
If Thunderbird has problems afterwards
Record the exact error, account type and time. Try Troubleshoot Mode from Help, choosing only temporary continuation, not permanent resets. If the problem disappears, an extension or customization is a candidate, not yet a proven cause. If it remains, comparing with webmail may help distinguish account and client problems. Do not delete the profile or account speculatively or force a downgrade with the working profile. Preserve the current state before restoration too; consult official help or IT with redacted error details for the next step.
Sources and context
Editorial selection based on current vendor information; no invented search volumes or in-house software test. This noncommercial guide contains no affiliate links.