Firefox actually purges history automatically. For instance, the oldest history I have on this browser right now is from January 2018. I found about this the hard way.
[1] https://web.archive.org/web/20151229082536/http://blog.bonar...
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=500239
I need to do a handful of experiments to see exactly how this interacts with Sync, even though I've (foolishly) already synced the important profiles. I'd hope that the cloud copies of the places database just keeps growing, but in any case, I'd rather combine them all offline anyway.
Anyway, thanks for the guide.