Enable Cookies in Firefox on iPhone
It is no longer possible to disable cookies in Firefox on iPhone! And so, unfortunately we can't give you a guide to show you how to change the Enabled/Disabled setting.
This seems to be an iPhone/iPad specific thing for Firefox; the Cookies setting is still available on Desktop (Windows/macOS/Linux) as well as Android etc. So the fact that it's not available on iPhone or iPad is presumably something to do with Apple's fairly strict terms and conditions around developing apps for iOS.
Third-Party cookie settings are removed as well
This is unfortunate because they also seem to have removed the option to control Third Party Cookie settings.
Cookies in a "First-Party" context are generally pretty crucial to using modern websites, and from some perspectives disabling them maybe causes more problems than it solves, so maybe you can understand removing that option... However completely removing the ability to control third party cookies - which are far more often used by companies (such as Facebook) to track users on other websites is puzzling and frustrating.
If any Firefox iOS developers read this and can share some insight, we'd love to know!
At least you can keep clearing your cookies
There is at least the option to clear cookies in Firefox on iPhone.