As per the arkenfox wiki, you should not touch user.js but rather in user-overrides.js add (or create it) the following:

/* override recipe: enable session restore ***/ user_pref("browser.startup.page", 3); // 0102 // user_pref("browser.privatebrowsing.autostart", false); // 0110 required if you had it set as true // user_pref("browser.sessionstore.privacy_level", 0); // 1003 optional to restore cookies/formdata user_pref("privacy.clearOnShutdown.history", false); // 2811 // user_pref("privacy.cpd.history", false); // 2820 optional to match when you use Ctrl-Shift-Del

See info on overrides and the recipe to enable session restore

There is also Abigail at Philosophy Tube, she has this old video that while not extremely in depth is quite easy to digest.

Edit: Oh, and Zoe Baker in general for longer essays!