IcoFX is described as 'Professional icon and cursor editor designed to work with small images. You can easily create, edit, read and save icons with transparency for Windows XP, Vista and Macintosh. You can convert your favorite images into icons, or icons to images' and is a very popular Icon Creator in the photos & graphics category. There are more than 25 alternatives to IcoFX for a variety of platforms, including Windows, Web-based, Linux, Mac and Android apps. The best IcoFX alternative is Greenfish Icon Editor Pro. It's not free, so if you're looking for a free alternative, you could try Greenfish Icon Editor Pro or iconPRO. Other great apps like IcoFX are RealWorld Icon Editor, Iconion, Pixelformer and Yesicon.
You can achieve exactly the same results using Greenfish as you could with IcoFX, and any differences in their functionality are not noticable. The UI is a bit outdated but it is being actively updated and is still perfectly functional and logically laid out.