Sounds that older history has been deleted. Once you will move to the next month then older history will appear automatically. current Firefox releases have no time limit for the history, there is only a maximum for the number of history visit as found in the read-only pref named. You can find the current value of the maximum history visits as the value of the read-only pref places.history.expiration.transient_current_max_pages on the [/kb/about-config-editor-firefox about:config] page.