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