In order to change your Firefox Configuration please do the following steps :

1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear.
2. Click I'll be careful, I promise! to continue to the about:config page.
3. search for history

places.history.expiration.transient_current_max_pages double click and increase the number.

However, the larger it gets will affect the performance of Firefox. I would recommend using a third party back up service for your profile history.

Do you have a large history or do you have cleanup software that might be removing the history?

The places.history.expiration.transient_current_max_pages is a read-only pref, so changing its value won't have effect.

Current Firefox versions keep a maximum number of visited page and do not have a time limit for the history. Firefox determines automatically how many pages can be kept without affecting the performance. You can see the current value of this maximum as the value of the read-only pref places.history.expiration.transient_current_max_pages on the about:config page.

See Marco Bonardo's blog about this subject:

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