NSS SQLite-based DB
In NSS 3.35 (which corresponds to Firefox 58), NSS moved to default to using a SQLite based database.
Previously, NSS used the DBM file format by default. Starting with version 3.35, NSS uses the SQL file format by default.
When using the SQL type (either explicitly, or because of the new default), with a database directory which already contains a DBM type database, NSS will automatically perform a one time migration of the information contained in the DBM files to the newer SQL files. If a master password was set on the DBM database, then the initial migration may be partial, and migration of keys from DBM to SQL will be delayed, until this master password is provided to NSS. (Conversely, NSS will never synchronize data from SQL to DBM format.)
Details on the database information can be found on this Fedora Linux project page: https://fedoraproject.org/wiki/Changes/NSSDefaultFileFormatSql