The 'f' is really ugly, and the * not being centered is a shame (it's used so much in source code for ⨉, it really should be vertically centered), but the reduced number of serifs is nice. I also like how === has less spacing between the lines, which fits common source code conventions which use = for underlines and horizontal rules.

Here's the same with Source Code Pro: http://screenshotcomparison.com/comparison/140474

I'm liking the asterisk and kerning in general. It's taller, so it feels a little more cramped between lines. That might be a dealbreaker for some.

I think you've made a mistake. The a glyph in your demo doesn't look like the glyph in the Github repository...

Is it just me or is screenshot comparison useless on mobile?

Fira: https://imgur.com/nbzZsxi

Hack: https://imgur.com/X2qLx8H

On my Mac, Hack has increased leading (space between lines) which makes it more readable. Quite a bit more IMO. I'm curious why your screen shots show identical leading.

Compared to Fira or compared to Hack rendered on my system? Are you using Atom/WebKit too? Same font size?

Hack has greater leading compared to Fira. I notice it when viewing in either Xcode, TextMate or Android Studio.

Whoa, really? Fira already has a much larger line-height than nearly any other mono font I've used.

They've reduced the line-height quite a bit in one of the more recent versions.

Fira at low res without antialiasing and without retina display looks right. I'd choose fira.