Might be because the browser history is limited by default: https://support.mozilla.org/en-US/questions/1039372

Details on the ranking algorithm: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/Places...

Thanks! That might be it.

For future notice though, the answer in the first link is wrong; you want "places.history.expiration.current_max_pages", not "places.history.expiration.transient_current_max_pages". Apparently the max "pages" are automatically calculated by default- mine was around 160k, so my history only went back about 2.5 years. I've set it to the maximum value for a 32 bit integer, which I guess should fix the problem for the next 20 or so years.

Kind of annoying that Firefox's default is to delete things when you told it not to delete things.

They do it for performance reasons, but I've not noticed any slowdowns yet at least. Still - would be better if they had a limit on how long back they searched instead of actually deleting data. That would most likely be more work ofc.