Tried to get it at least readable for 15 mins or so and gave up: http://paste.click/msFBVy

Tried all kinds of variations of

    > fnt="xft:Fira code:pixelsize=10:size=15"; urxvt -fn "$fnt" -fb "$fnt" -fi "$fnt" -fbi "$fnt" -letsp -1

Can't get the font spacing to not be massive and the ligatures don't work.

My build is:

    rxvt-unicode (urxvt) v9.21 - released: 2014-12-31
    options: perl,xft,styles,combining,blink,iso14755,unicode3,encodings=eu+vn+jp+jp-ext+kr+zh+zh-ext,fade,transparent,tint,pixbuf,XIM,frills,selectionscrolling,wheel,slipwheel,smart-resize,cursorBlink,pointerBlank,scrollbars=plain+rxvt+NeXT+xterm

Edit: Tried hasklig that was mentioned. Ligatures still don't work but the spacing is fine: http://paste.click/cEWQZs