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>

agent/BACKLOG.md

@@ -198,50 +198,6 @@ scaled splash — the LUKS prompt especially, since a passphrase box half off
 the panel is the failure that matters. Pass = docked boot + shutdown centred
 on both heads; undocked unchanged.
 
-### 144. MIGRATION.md must warn about Mozilla `profiles.ini` — it reads as total data loss
-
-Bernardo 2026-07-16: "it seems that I have lost all my thunderbird accounts and
-emails!" — nothing was lost. `~/.thunderbird/bernardo` was intact the whole
-time: **30 GB**, 64 account/identity prefs, 19 GB `ImapMail` + 9.7 GB `Mail`,
-last used 2026-06-26. Only `profiles.ini` was gone, so on first launch after the
-migration Thunderbird did what it always does with no profile list — created an
-empty one (`wc966grk.default`, 16 MB) and pointed itself at it. Fixed by
-rewriting `profiles.ini` to `Path=bernardo` + `Default=1` (old file kept as
-`profiles.ini.bak-*`); Thunderbird reopened the real profile and went straight
-into its Duo login. **Total fix: one 8-line file.**
-
-This is the **same class** MIGRATION.md's "Browser profiles" bullet already
-covers — a file Home Manager used to manage disappears, and the app reads its
-own absence as "first run" — but that bullet only names Chromium extensions.
-The Mozilla case deserves its own warning precisely because it is *louder and
-less dangerous*: Chromium silently deletes extension data (real loss, looks
-minor); Thunderbird touches nothing and merely looks away from 30 GB (no loss,
-looks catastrophic). A user who believes the second one will go restore a
-snapshot over a perfectly good profile — the recovery is the risk here.
-
-Firefox on this box shows the same fingerprint, unreported because he browses in
-Chromium: `~/.mozilla/firefox/profiles.ini` (Jul 12) points at
-`u9p8fdhh.default` (101 MB, created at migration), while
-`profiles.ini.hm-bak` — Home Manager's own backup, dated **Jun 2025** — points
-at `lfr4aapb.default` (129 MB, last used 2025-09-07), and a third profile
-`default` (290 MB) was last used 2026-03-02. So the pattern is Mozilla-family,
-not Thunderbird-specific. **Do not repoint Firefox without asking Bernardo** —
-u9p8fdhh has been in use since Jul 14 and merging profiles is his call.
-
-**What to write** (docs only, no code): in MIGRATION.md's Phase-0/browser
-section — before first launch of any Mozilla app, check that
-`~/.thunderbird/profiles.ini` and `~/.mozilla/firefox/profiles.ini` exist and
-name your real profile directory; if an app opens empty, **do not restore a
-snapshot** — `ls ~/.thunderbird` first, and if a fat profile dir is sitting
-there, it is a one-file fix. Include the working `profiles.ini` shape and the
-`Name=`/`Path=`/`Default=1` semantics.
-
-**Root cause is unproven and probably not worth chasing:** zero HM generations
-survive on disk to show whether HM ever managed that file (the Firefox
-`.hm-bak` proves it did there, in 2025), and the snapshot check needs sudo. The
-warning is worth writing either way — it costs nothing and it is the difference
-between a one-line fix and a restore that overwrites good data.
-
 ### 143. Rofi keeps the selected *row number* when the search changes, so the highlight lands on an unrelated entry
 
 Bernardo 2026-07-16, and his repro is exact: open the app menu, type `ca`, move