The received video works, but the forwarded copy is incomplete? Thunderbird 159.0 corrects an MP4 forwarding defect. This guide helps desktop users and small offices separate the original, draft and transfer stages without deleting messages or attachments. [1]
Which problem does this cover?
Bug 2077839 describes an intact MP4 attachment becoming truncated during normal forwarding. This differs from an attachment dialog that will not open, a blank PDF tab or a mail server size limit. The tracker marks Thunderbird 159 fixed; this is not blanket approval of every ESR version. Patch approval in a tracker does not by itself prove that an ESR release has shipped. [2]
Preparation and backup
You need the original message, sufficient local storage and a trusted test video containing no confidential information. Keep the original untouched and back up the profile before updating. Mozilla limits the built-in ZIP export to 2 GB; use the documented folder-copy method for larger profiles and fully close Thunderbird before copying. Store the backup separately with restricted access. [3]
Five safe checks
- Check the original: Save the received MP4 attachment into a separate test folder without overwriting an existing file. Record its actual size in bytes and check the beginning, middle and end in an up-to-date local player. If this file is already damaged, forwarding cannot be the sole explanation.
- Compare the draft: Reopen the original message and select Forward without sending it. Look for a missing attachment or a noticeably smaller size indication. Rounded KB/MB figures are only clues: encoding and headers make the whole email larger, so its size is not directly comparable to the MP4 file. Keep the original and draft separate.
- Use a control path: Create a new draft and manually attach the complete MP4 saved earlier. This workaround, described in the bug report, avoids the normal forwarding routine but does not guarantee success. Recheck the filename, attachment and recipients; no sending is needed for local diagnosis. [2]
- Update deliberately: After backing up, record the full version number and release channel. Use an approved release with a documented correction; 159.0 explicitly lists this fix. About Thunderbird may start downloading updates. Update distribution-packaged Linux versions through their package manager; consult IT for managed installations and do not switch ESR channels casually. [1][4]
- Repeat the same case: After restarting, forward the same untouched original again and compare it with the new draft containing the manually attached file. If a complete transfer test is necessary, use only an approved test mailbox and non-confidential data. Compare the MP4 saved there with the local original, not the size of the entire email.
Acceptance and recovery
Acceptance requires the saved recipient copy to have the same byte count and play completely; comparing SHA-256 values can additionally confirm byte equality. Matching size alone does not prove identical contents, and playback alone does not prove byte equality. Without sending, only the local draft check is complete. If results differ, do not send the problematic draft, retain the original and backup, and use the controlled manual attachment path. Avoid uncontrolled downgrades or profile resets.
Limits, date and sources
As of 3 November 2026. This is source-based guidance, not our own Thunderbird reproduction test. For persistent failures, give support the version, operating system, forwarding mode and byte counts; sanitize messages and videos first. Never upload original mailboxes or confidential attachments publicly. Do not disable security software. The specific release statement concerns Thunderbird Desktop; check other clients, mobile apps, server limits and ESR delivery separately. No shopping or affiliate links.