The Firefox OS' Fira typeface doesn't contain some specific characters
for Vietnamese and the system has to fallback to another typeface for
those characters. This can be seen in the following images:
Home screen:
http://i.imgur.com/ZQKknFd.png
Settings:
http://i.imgur.com/7sMa6If.png
@arky: in which project/component should I file this bug?