You may have cleanup software running on your computer if older history gets removed.<br /> With build-in Firefox means you can only remove recent history and not older history unless you do this manually in the History Manager (Library) *History > Show All History Firefox 4+ versions do not have a time limit for the history.<br /> Firefox determines automatically how many pages can be kept without affecting the performance.<br /> You can see the current value via the read-only pref places.history.expiration.transient_max_pages on the about:config page.<br /> See Marco Bonardo's blog about this: * http://blog.bonardo.net/2010/01/20/places-got-async-expiration