Firebase uses last write wins but they do have a backlog so the user can sync up. There is also a transaction () function for manual conflict handling. Perhaps you can automatically raise an exception in the event of conflicts.
https://stackoverflow.com/questions/19975921/how-does-fireba...
A bit of feedback, the API looks fairly verbose compared to Firebase so a high level wrapper would be nice. Though that could simply be a side effect of Next.js being clunky.