This is a long running problem which I think started in autumn 2014. I suddenly lost lots of history I'd relied on to be able to find pages I half remembered. Settings still the same. Nothing else notable appeared to have happened to the machine at the time.
In 2015 the machine got a browser redirect infection, and eventually I ended up having to remove Firefox and all the hidden program files and things associated with it. I thought the reinstalled version might start doing as it was told with the history, but it never has. Only remembers that day's usage and won't keep it beyond that.
Which has its advantages for privacy perhaps, but it means that if there's anything I think there's the slightest possibility I want to find again, it has to go in favourites, and you never quite know at the time.