migrateprofile.py - Set Firefox or Thunderbird to use the profile.

[Poikilos]

Firefox

Final ~/.mozilla/firefox/profiles.ini (after adding Profile2 then opening Firefox and choosing it in about:profiles):

[Profile2]
Name=user
IsRelative=1
Path=ifq13mzu.default

[Profile1]
Name=default
IsRelative=1
Path=vt5qwe6r.default
Default=1

[Profile0]
Name=default-release
IsRelative=1
Path=kzha1hmh.default-release

[General]
StartWithLastProfile=1
Version=2

[Install11457493C5A56847]
Default=ifq13mzu.default
Locked=1

installs.ini isn't affected:

[11457493C5A56847]
Default=ifq13mzu.default
Locked=1

[Poikilos]

Here are the old computer's profiles (on 10.13.6 High Sierra).

Firefox 91.0.2

via Owners-Mac-mini:~ user$ cat Library/Application\ Support/Firefox/profiles.ini:

[Profile0]
Name=default
IsRelative=1
Path=Profiles/ifq13mzu.default
Default=1

[General]
StartWithLastProfile=1
Version=2

[Install2656FF1E876E9973]
Default=Profiles/ifq13mzu.default
Locked=1

installs.ini:

[2656FF1E876E9973]
Default=Profiles/ifq13mzu.default
Locked=1

Thunderbird 60.5.1

(updated macOS' Thunderbird to 78.13.0 after the migration)
via cat ~/Library/Thunderbird/profiles.ini:

[General]
StartWithLastProfile=1

[Profile0]
Name=default
IsRelative=1
Path=Profiles/c49m8v3i.default
Default=1