NOTE:  Cookies are automatically enabled in Firefox 56. 

Step 1.  Click the Menu  button and select Content Blocking.

Step 2.  Select the Privacy & Security panel. This is where you can view your settings for Content Blocking, which includes cookies. 

If standard is selected, it means that you are using the default settings for content blocking and cookies are enabled. To block cookies, continue to Step 3.

Step 3.  To enable all cookies, do one of the following:

• Select Custom and clear the Cookies check mark.
  
• Alternatively, select Standard to restore the default settings.