Enable Cookies in Firefox on iPhone
The Mozilla team have removed the ability to block/allow cookies in Firefox on iPhone! Firefox is now set to always accept cookies and this cannot be changed. 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. It's very strange that they've removed that option just for Firefox on iOS.
Third-Party cookie settings are removed as well
This is unfortunate because Mozilla have also 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 and forcing users to always accept cookies, to save them from getting confused... 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.
Firefox have added a "Tracking Protection" option in the Settings menu, which sounds like it might control whether third-party cookies are accepted or not; however the User Interface for this section is overly simplistic and doesn't include anything helpful to let you know exactly what it does or doesn't do.
Even when you enable "Strict" mode, it still just says "Blocks known trackers"; which implies that it still may let third-party cookies that it doesn't have on it's "block" list to be set.
Google Chrome has also removed this option for their browser on iOS; it's a strange trend!
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.