Load Thunderbird's mail folders (mbox) straight from your profile and save every email as a PDF with attachments and index.csv. No add-on needed, nothing uploaded.
Thunderbird keeps each folder as an mbox file inside your profile: Inbox, Sent and so on, with no extension, and subfolders in Inbox.sbd. Mailtrail reads those files directly and skips the .msf index files.
How to do it
In Thunderbird, open Help → Troubleshooting Information and click Open Folder next to Profile Folder.
Go into Mail/Local Folders (POP and local mail) or ImapMail/<server> (IMAP; only messages Thunderbird has downloaded for offline use are there).
Close Thunderbird (so the files are not being written to), and copy that folder somewhere convenient.
Open the converter in Chrome, Edge, Firefox or Safari. Nothing is installed and nothing is uploaded. Click Choose a folder and pick the copied folder. The Thunderbird folder tree appears in the folder filter.
Pick the outputs: one PDF per email (on by default), an attachments folder, optionally one merged PDF, .eml copies and .mbox per folder. Click Export and choose where to save the ZIP.
Tips
IMAP accounts: right-click the account → Properties → Synchronization & Storage → tick Keep messages for this account on this computer and let it download first.
Deleted messages Thunderbird hasn't compacted away may still be in the mbox file; we skip messages marked as expunged by Thunderbird's X-Mozilla-Status header.
What you get
One PDF per email with a header block (subject, from, to, cc, date, folder, attachment list) and the body, including embedded images. Searchable text, not screenshots.
Your folder structure mirrored in the ZIP, and file names that sort by date: Inbox/2024-03-05_0912 Jane Doe - Invoice March.pdf.
Attachments as original files under attachments/.
index.csv (date, from, to, cc, subject, folder, file names, attachment count, Message-ID, pages) and errors.csv for anything that couldn't be converted.
Optional: one merged PDF, .eml per email, .mbox per folder. Pro adds Bates numbering, page numbers, SHA-256 hashes and bookmarks.
Private by design
The conversion runs inside your browser tab with Web Workers. Your archive is never uploaded, and there is no server that could see it. You can even disconnect from the internet after the page loads. The only network call is the optional license-key check, which sends the key and a random device ID.
Questions
Do I need ImportExportTools NG?
No. That add-on can also print to PDF from inside Thunderbird, but for large folders the converter is faster and also gives you attachments and an index.
What does it cost?
Free for up to 25 emails per export. Standard ($29 one-time) removes the limit; Pro ($69 one-time) adds Bates numbering, page numbers, hashes and bookmarked merged PDFs. See pricing.