Using an extension cord can be safe and it can be a fire hazard. If the ends of the cord become dirty or corroded they will have resistance which will generate heat. The higher the current and/or resistance the higher the heat. If you want to get technical about the ampacity of the wire you intend to use Ugly's Electrical Reference
is a great reference with a lot of no BS info to help you determine what gauge extension wire will be safe for your situation. I provided a link to amazon but I think some of the big box hardware stores like home depot carry it too. I realize it's not directly applicable to an extension cord with the FFE but when I used to work with industrial equipment we had to pull the coiled power cords out of their box for the servo motors while testing/debugging. If we didn't the powers cords would over heat, melt, and short out. While not likely, I do think it would be possible to have a scenario where an extension cord coiled/balled up and enclosed such that it cannot transfer heat to the surrounding air could get hot enough to melt.
Probably the best thing you can do to avoid a problem with an extension cord is to not use one. If that's not an option, this is a place where you want to spend more money for a quality cord and get the lowest gauge (thickest, 10ga min.) wire you can. Don't buy a cord any longer than you absolutely need. To be extra safe feel the cord near the ends after plugging in. If they quickly become hot, unplug from the car asap and don't use that cord. You probably should also periodically check the cord for heat while charging. It's not unusual for the cord to become warm just make sure it doesn't get hot. Yes I know that last part is vague but this is where you have to use your grey matter to decide what is and what isn't acceptable when you are using your laser precision temperature measurement devices at the end of your arms <sarcasm> Suffice it to say, slightly warm = ok and hot = bad!
From my personal experiences about a year ago, I used the book mentioned above as a guide to determine kind of extension cord I needed. I determined a 10ga 40' extension cord would be safe for me. I don't remember the exact details but I think a 12ga would have been sufficient but I went a size or two thicker than what I needed for an added margin of safety. I've only used it a hand full of times and have had no issues. I've never need anywhere near the full 40' but made sure the coiled up remaining wire was spread out enough to dissipate heat should it get hot. As an added safe guard and since charging time wasn't an issue, I set my EVSE to charge at 8A instead of 12A that the included Ford EVSE will pull. Oh yeah, In case you're wondering, you won't find a cord rated higher than 15A even though the wires inside may be capable of higher current simply because the connectors on the ends are only rated at 15A and also the outlets are 15A max as well.
I can tell you from experience to avoid the Harbor Freight cheapies unless you want to start a fire. I had an old cord of unknown gauge that I got from harbor freight years ago. I didn't expect it to work but I wanted to experiment and see what would happen. It was a short experiment as almost immediately the cord near the ends became too hot to touch. Had I been stupid enough to try a full charge, I likely would have burned down my house. This is why they don't want you to use an extension cord since inevitably some idiot would burn their house down and blame Ford for their own stupidity.