docs(migration): #144 — Mozilla profiles.ini, and why NOT to restore a snapshot

A real migration (2026-07-16) lost ~/.thunderbird/profiles.ini, so
Thunderbird made an empty profile and opened it while 30 GB of mail —
19 GB ImapMail, 64 account prefs — sat in the directory it had stopped
looking at. The fix was eight lines.

MIGRATION.md already warned about this class for Chromium extensions.
The Mozilla case gets its own bullet because it is the inverse failure
and the inverse danger: Chromium silently deletes extension data and
looks minor; Thunderbird loses nothing and looks catastrophic — so the
reflex it provokes, restoring a snapshot over a good profile, is the one
thing that can actually cost you mail. The bullet therefore leads with
`ls`/`du` ("a multi-GB dir = your data is fine"), then gives the working
profiles.ini shape and the Path=/Default= semantics.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/MIGRATION.md

@@ -289,6 +289,47 @@ default yet.
   `pre-nomarchy-home` snapshot, then reinstall each extension — from
   the Web Store (ids are stable, so the data reattaches) or by
   re-declaring the ids if you want them declarative again.
+- **Thunderbird / Firefox — “all my email is gone” (it isn’t):** Mozilla
+  apps find your data through **one small text file**,
+  `~/.thunderbird/profiles.ini` (and `~/.mozilla/firefox/profiles.ini`),
+  which names the profile directory to open. Lose that file and the app
+  does not error — it does what it does on a brand-new machine: creates
+  an empty profile and cheerfully opens *that*. Every account, folder and
+  message is still on disk, in the profile dir it stopped looking at.
+  This bit us on a real migration (2026‑07‑16): a 30 GB profile with 19 GB
+  of `ImapMail` went “missing”; the fix was eight lines of `profiles.ini`.
+  **Before first launch,** check the file exists and names your real
+  profile:
+  ```console
+  $ ls ~/.thunderbird/            # bernardo/  ← the fat one is your profile
+  $ cat ~/.thunderbird/profiles.ini
+  ```
+  **If an app opens empty, do not restore a snapshot** — look first:
+  ```console
+  $ du -sh ~/.thunderbird/*/      # a multi-GB dir = your data is fine
+  ```
+  If a fat profile is sitting there, this is a pointer problem, not data
+  loss. Close the app, then write (`Path=` is the directory name, relative
+  to the `.thunderbird` dir):
+  ```ini
+  [Profile0]
+  Name=default
+  IsRelative=1
+  Path=<your-profile-dir>
+  Default=1
+
+  [General]
+  StartWithLastProfile=1
+  Version=2
+  ```
+  Keep the empty profile listed as `[Profile1]` (no `Default=`) if you
+  want it out of the way rather than deleted. The **restore is the risk
+  here**, not the bug: rolling a snapshot over a good 30 GB profile to
+  “recover” data that was never lost can cost you the mail that arrived
+  since. Same shape as the Chromium bullet above — a file HM used to
+  manage disappears and the app reads its own absence as “first run” —
+  but inverted: Chromium *deletes* quietly, Thunderbird *loses nothing*
+  and looks catastrophic.
 - **VPN:** NetworkManager connections survive under
   `/etc/NetworkManager` and your home. System › VPN lists NM
   `vpn`/`wireguard` profiles; import any that lived outside NM.

docs/ROADMAP.md

@@ -449,6 +449,17 @@ Design/decision records and a running log of shipped work (items marked
   (#131). Those windows therefore carry float/center rules and deliberately no
   `size` rule. Measured after: calendar 1536×936 = exactly 60%×65%, doctor
   1408×1008 = 55%×70%, both centred in the Waybar work area.
+- ✓ **MIGRATION.md: Mozilla `profiles.ini` (#144, 2026-07-16):** a real
+  migration ate Bernardo's `~/.thunderbird/profiles.ini`, so Thunderbird did
+  what it does on any new machine — made an empty profile and opened it —
+  while 30 GB of mail (19 GB `ImapMail`, 64 account prefs) sat untouched in the
+  directory it had stopped looking at. Recovered in eight lines. Documented as
+  its own bullet next to the Chromium one because it is the **inverse failure**
+  and the inverse danger: Chromium silently deletes extension data and looks
+  minor; Thunderbird loses nothing and looks catastrophic — so the reflex it
+  provokes (restore a snapshot over a perfectly good profile) is itself the way
+  to actually lose mail. The doc therefore leads with `ls`/`du`, not with a
+  restore.
 - ✓ **Ask hands off to a web chat (#140, 2026-07-16):** Bernardo: "opening
   claude-code is too disruptive." SUPER+CTRL+A used to spend a terminal, an npm
   fetch and a REPL on "what's the syntax for…" — the prompt was the right