Bug#800143: Fira Sans webfont is blank

[VA]

Package: iceweasel
Version: 38.3.0esr-1

When I open https://www.google.com/fonts/specimen/Fira+Sans in 
iceweasel, blank glyphs are shown for this web font. Other web fonts 
work correctly: https://www.google.com/fonts/specimen/Crete+Round 
displays fine, for example.

Note I observe the same behavior with chromium, but I don't know what's 
the culprit so I assign it to iceweasel until somebody has more clues 
than me.