Check your privacy settings. Maybe Firefox is deleting your history on closing (or not remembering history in the first place)

Regarding the above post:

"3-bar" menu button (or Firefox menu) > Preferences

In the left column, click Privacy. On the right side, do you have:

• Firefox will: Remember history
  • In this case, it should work

• Firefox will: Use custom settings for history
  • Make sure there is a checkbox for Remember my browsing and download history
  • Uncheck Clear history when Firefox closes or if you use that, click the Settings button to the right and make sure Browsing & Download History is not checked

You can also check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox