It's an extension of the Fira Mono font containing a set of ligatures for common programming multi-character combinations. The font looks awesome paired with the right color set. Also, the editor and terminal support is great.
Yeah Fira Code makes life so much better. I was also worried about getting too used to the ligatures but that's like saying you won't use the Internet because you don't want to get used to the convenience. It's a great tool to increase your productivity, what more do you need to know!
Agreed, I gave a try to ligature fonts a few years ago... wasn't convinced. Then tried it again last year, for ligature-riche languages (Scala and R) .... and really felt in love with FiraCode... once used to it, it makes things so much more readable!
@gskema - Jetbrains added support for ligatures fonts in their editors, including PhpStorm, in their ver. 2016.2 and later. You need to enable it from Settings → Editor → Colors & Fonts → Font → Enable Font Ligatures.