In about:config, you could try setting places.history.expiration.max_pages to 9999999 (or very large number of your choice).

Without this setting, Firefox tries to be "smart" and won't allow your history to occupy more than a certain percentage of your disk space. This caused a problem for me because I have my profile on a dedicated RAMDisk. However, if you're on a 1.8TB hard drive, there's probably something different going on in your case.

Profile data can be removed or not recorded in several ways;

• 1) Running Firefox in Private Mode (nothing is recorded)
• 2) Privacy settings; (In the address bar, type about:preferences#privacy<enter>)
• 3) Many disk cleaners and anti-virus type programs can remove data from browsers
  Open their settings and have them leave the browsers alone.
• 4) Mal-ware.

You may be in Private Browsing mode or told Firefox not to remember.

Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings.

Turn off Always Use Private Browsing Mode Turn on Remember My Browsing And Download History At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

Macs: Make sure you install Firefox properly and that you do not run Firefox from within the DMG (Disk Images) file. https://support.mozilla.org/en-US/kb/how-download-and-install-firefox-mac