There is no time limit for the history, only a maximum for the number of entries in the . If you are below this limit and lose history then something else must be removing older history. Note that Sync only stores a limited amount of history if you use that feature. You can see the current stats if you click "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page. Current Firefox versions keep a maximum number of visited page and do not have a time limit for the history. Firefox determines how many pages can be kept without affecting performance. The places.history.expiration.transient_current_max_pages pref is a read-only pref that keeps the current value of this calculated maximum number of visited pages. You can create a new Integer pref named places.history.expiration.max_pages if you want to modify the automatically calculates number of pages as shown in places.history.expiration.transient_current_max_pages.