Release 2 · Thunderbird add-on
MSG Opener for Thunderbird
A free Thunderbird add-on that opens Outlook .msg files as normal messages, with their attachments, on your own machine.
Status
Built and tested; not yet on the Thunderbird add-on site. We’re preparing the submission. We’ll add the store link here when it is listed. Until then, the source is on GitHub.
What it does
Thunderbird can’t open Outlook .msg files on its own. This add-on converts a .msg to a normal message and opens it in a tab, with its attachments and any messages embedded inside it.
- Right-click a .msg attachment and choose “Open as message”.
- Or click the Open .msg toolbar button and pick one or more .msg files, or drop them on the page. “Import into folder” keeps a copy in a folder you choose.
- When a detail differs from the original, such as a sender with no email address in the file, a bar at the top of the message says so.
- Everything runs on your machine. The add-on asks for no network access and collects no data.
It needs Thunderbird 140 or later. The conversion is done by msg-eml-core, our open-source library.
Versions tested
The QA agent ran the add-on in two versions of Thunderbird, both on Linux.
| Thunderbird | Platform |
|---|---|
| 140.17.0 ESR | Linux |
| 157.0.1 | Linux |
Not tested: Windows, macOS, and any other Thunderbird version.
QA verdict: SHIP
The QA agent was a separate Claude session and didn’t build the add-on. It tested the exact file we built, msg-opener-for-thunderbird-0.1.0.xpi, SHA-256 3f129d040af8566f3749ec6fd58c5d9dc3f8fe9f4765f56fd5a50874321c68a8. The verdict was SHIP, on 2026-10-08.
The QA reports are internal and not published, so there is no link for the claims below. This section is summarised from our internal QA notes:
- Corpus: 65 .msg files, plus three extra test files built to check the warning bar. 63 of the 65 opened. The other 2 are deliberately damaged files, and the add-on refused both with a message that says what is wrong. 0 crashes, hangs or blank tabs. The results were the same on both Thunderbird versions.
- Embedded messages: messages inside other messages opened in all 9 cases, across 7 files.
- Import: importing a message twice is detected and refused.
- Bad input: a zip and an .eml renamed to .msg, and an empty file, each gave a clear message and no tab.
- Size: a 20 MB file opened from disk in 2 to 3 seconds. A 26 MB file was refused at once, with a message.
- Earlier problems fixed first: the first QA pass found that the right-click entry could stop appearing after you selected a file that wasn’t a .msg. That was fixed, and three later passes checked it in the message tab, in the three-pane view and after a restart. The later passes also reworded every message the add-on shows, after a writer’s review, and checked the new wording.
- Corpus: synthetic and public files only, no real mailboxes.
Known limitations
- It needs Thunderbird 140 or later. It was tested on 140 ESR and 157, on Linux.
- A sender or recipient with no email address in the file is shown with a placeholder address ending in
.invalid. Replies to it go nowhere. - A file with no date gets no Date header. Where a file has no sent time, the date shown is when it was delivered or created.
- Rich-text (RTF) bodies are shown as plain text, with the formatted version attached.
- Calendar items, contacts, tasks and notes open as ordinary messages.
- Damaged files are refused with a message. Files over 25 MB are refused.
- It opens .msg files only. It does not open .oft templates, .pst files or winmail.dat. Encrypted .msg files were not tested.
- An opened message is a view of the file. To keep it, use “Import into folder”.
- A large .msg attachment (20 MB) took 7 to 8 seconds to open from the right-click entry, and Thunderbird may pause briefly while it does.
Launch targets, set before launch
We set these targets before release, so the results can’t move them. They count the add-on’s daily users on the Thunderbird add-on site, from the day the listing goes live.
| Checkpoint | Result | What it means |
|---|---|---|
| Day 30 | Under 4 daily users | Below the median for new add-ons. |
| Day 30 | 11 or more daily users | Top quartile. |
| Day 90 | Under 16 daily users | Median. We stop maintaining it beyond security fixes. |
| Day 90 | 59 or more daily users | Top quartile. We continue. |
| Day 90 | 161 or more daily users | Top decile. We consider adding winmail.dat and .olm support. |
| Any time | A rating under 3.5 | We fix it or withdraw it. |
Results
Not yet. The clock starts when the listing goes live. We’ll publish the numbers here, whether we hit the targets or miss them.
Licence and source
- Licence: Apache-2.0.
- Source: github.com/acalebratliff/msg-for-thunderbird.
- Converter: github.com/acalebratliff/msg-eml-core.
- Written and tested with AI assistance (Claude). The author reviews and publishes.