Usage of Private Use zone(0xE000 - 0xF8FF) in "UnDotum" font causes Font Awesome icons rendered wrongly in Mozilla Firefox

[Ｖ字龍(Vdragon)]

I found that Font Awesome icons on http://data.gov.tw is wrongly rendered on Firefox but not in Chrome:
(Screenshot attatched)

According to the Font Awesome's page of one of the icons and some Google research it seems that those characters are in 0xE000 - 0xF8FF : Private Use.

I found out that it's UnDotum font that provides the glyph, is it really necessary to assign glyphs in that area?

### Producing environment

    Ubuntu 14.04LTS
    Mozilla Firefox 43.0
    Google Chrome 47.0.2526.106
    fonts-unfonts-core 1.0.3.is.1.0.2-080608-10ubuntu1 from Ubuntu 14.04 software repository

### Upstream bug report
http://kldp.net/projects/unfonts/issue/318678

Thanks in advance!

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: fonts-unfonts-core 1.0.3.is.1.0.2-080608-10ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-74.118-generic 3.13.11-ckt30
Uname: Linux 3.13.0-74-generic i686
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: i386
CurrentDesktop: Unity
Date: Tue Jan 5 18:51:09 2016
InstallationDate: Installed on 2015-10-02 (94 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release i386 (20140417)
PackageArchitecture: all
SourcePackage: fonts-unfonts-core
UpgradeStatus: No upgrade log present (probably fresh install)

[Changwoo Ryu (cwryu)]

It's not a "bug" of this font. PUA can be used in any way by any fonts. This problem is because Un-fonts precedes Font Awesome in your fontconfig configs.