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

uBlock Origin is really all you need for blocking ads, if you also wanna deal with the whole sponsors on videos SponsorBlock is a good companion!