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 read-only pref places.history.expiration.transient_current_max_pages pref 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.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

01.Go to the Firefox menu and select Preferences.Then select the Privacy & Security tab to the left.Next, locate the History section.

02.The first option found in the History section is labeled Firefox will and is accompanied by a drop-down menu with the following three choices.

   -Remember history: When selected, Firefox records a log of all of your browsing history.
   -Never remember history: When selected, Firefox does not record any browsing history whatever.
   -Use custom settings for history: Allows you to customize a number of Firefox's history-related settings, detailed below.

03.As mentioned above, Firefox allows you to customize a number of its history-related settings. When Use custom settings for history is selected from the drop-down menu, the following customizable options are made available.

   -Always use private browsing mode: When enabled, Firefox will automatically launch in Private Browsing mode.
   -Remember my browsing and download history: When enabled, Firefox will maintain a record of all websites that you've visited as well as files that you've downloaded.
   -Remember search and form history: When enabled, Firefox will store most information that is entered into Web forms as well as keywords submitted to a search engine via the browser's Search bar.
   -Accept cookies from sites: When enabled, Firefox will save all cookies pushed by websites to your local hard drive.
   -Clear history when Firefox closes: When enabled, Firefox will automatically delete all history-related data components each time the browser application is shut down.

Source- https://www.lifewire.com/manage-browsing-history-and-private-data-in-firefox-4103604