To turn cookies on in Firefox, click the menu button in the top-right corner of the browser and select Settings. In the left menu, click Privacy & Security.
Under Enhanced Tracking Protection, make sure Standard is selected. This is Firefox’s default setting and allows cookies, except for known trackers. If Strict is selected, change it to Standard. If Custom is selected, either uncheck Cookies or set it to block only cross-site tracking cookies.
If a specific website still says cookies are blocked, scroll down to Cookies and Site Data, click Manage Exceptions, and make sure that website is not listed as blocked. Changes are saved automatically. Mozilla’s current help page lists this path as Menu → Settings → Privacy & Security, with cookie controls handled under Enhanced Tracking Protection.