I can't be sure. When I say 'combine them all offline', I mean using something like [1] which refuses to do anything for me because the Waterfox database version is a rather old Firefox version, and that seems to expect all the db's versions to be up-to-date and equal, which seems pointless. #include <sqlite3.h> was my next step-- only I don't walk very well, so that didn't happen "yet". Or I'm lazy, or distracted, or depressed, or something. When I recently got tired of realizing a thing was on the other machine, I bit the bullet and synced them, if only to see how well that worked.

Anyway, thanks for the guide.

[1] https://github.com/crazy-max/firefox-history-merger