Thank you, this is the only thing what slowed me down switching to Librewolf.

Seems that MySQL and Spanner are the only supported DB-Engines by now, but there are efforts to support sqlite and Postgres[1]. I really hope these get merged soon, would make it much easier to backup your whole sync storage contained in one single sqlite file. However, a multiuser instance would probably be better stored via postgres due to better parallel update support.

EDIT: After trying this out: How exactly do I force Firefox (Librewol) using the self-hosted URL? Is there an about:config setting?

1: https://github.com/mozilla-services/syncstorage-rs/issues/49...