There is no time limit in Firefox 4+ versions for the history.
Firefox determines automatically how many pages can be kept without affecting the performance.
You can see the current value via the pref places.history.expiration.transient_max_pages on the about:config page.

See:

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

Make sure that you do not use Delete browsing, search and download history on Firefox to clear the Browsing History.

If you use cleanup software like CCleaner then check the settings for the Firefox application.

This seems unacceptable. Don't we still expect our software to behave deterministically?

Does this apply to the number of items kept in download history as well?

• Expire history by days: https://addons.mozilla.org/en-US/firefox/addon/expire-history-by-days/