browser.history.maxStateObjectSize and places.history.expiration.max_pages (both accessible through about:config) should be the settings to modify. Do a web search for those properties to get more information on how they interact.

There's a good chance an about:config setting lets you change it. FF has a ton of detailed settings but most are hidden.

On my setup the "manage history" window goes back a bit over 6 months. Unfortunately the list entries don't show a date, so I can't give more precise numbers.