Firefox's Places subsystem does use SQLite. Firefox's built-in SQLite implementation is compiled with the SQLITE_SECURE_DELETE mechanism referenced elsewhere in this thread (https://dxr.mozilla.org/mozilla-central/source/db/sqlite3/sr...) and a configure check is performed when Firefox is built with system SQLite (https://dxr.mozilla.org/mozilla-central/source/configure.in#...).

Additionally, there's a unit test that verifies that the feature is really working at https://dxr.mozilla.org/mozilla-central/source/storage/test/... that checks that a string shows up in the database and then disappears from the database when the row is removed from the database.