Click on Tools>Options>Privacy and under the History section. I noticed in some versions of Firefox it is under Edit>Preferences>Privacy> and under History. Just select what you want to do with your browsing history there.

In the old days, Firefox pruned away old history based on age, but at some point it was changed to be determined more by available disk space and optimizing performance. According to this old thread, you can reduce the amount of entries kept, but not increase it (but I haven't tested that myself): How to tell FF4 to remember "infinite" history.

I suspect that attempting to keep all history forever would cause Firefox to slow down and eventually grind to a halt. Perhaps instead of keeping all that history in Firefox, you might be able to find an add-on or external tool to archive your history in some useful format. That really would depend on what you want it for.

See also Marco Bonardo's blog:

• http://blog.bonardo.net/2010/01/20/places-got-async-expiration