Are you clearing your recent history when you close Firefox?

Firefox stores App Tabs and tab group details in the same file that stores details of browsing sessions. Make sure that you have not set Firefox to delete browsing history. For details of the setting to check see https://support.mozilla.com/kb/Clear+Recent+History

I checked and it is sent to remember my history, so that's not the problem.

I am having the same problem, do you have a "Mac"? My history has not been cleared