Firefox doesn't correctly display color emoji because Dejavu includes b&w emoji

As seen on:

https://bugzilla.mozilla.org/show_bug.cgi?id=1516445
https://bugzilla.mozilla.org/show_bug.cgi?id=1323050

STR:

I am using Firefox nightly, but the installed Firefox built into ubuntu works as well. Firefox comes with a font "TwemojiMozilla.ttf" which includes emojis in color.

1. Go to https://en.wikipedia.org/wiki/Emoji#Unicode_blocks
2. Scroll to row U+1F60x

What happens:

The emoji are not displayed in color, because Dejvu Sans seems to be rendered instead of TwemojiMozilla (the font built into Firefox).

Expected result:

Color emoji. Since Dejavu doesn't display color emoji and Firefox was kind enough to include a font that includes color emoji, Ubuntu's fontconfig should let Firefox do the right thing.

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: fontconfig 2.13.1-2ubuntu2
ProcVersionSignature: Ubuntu 5.0.0-8.9-generic 5.0.1
Uname: Linux 5.0.0-8-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.10-0ubuntu25
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Tue Apr 9 00:32:37 2019
InstallationDate: Installed on 2018-10-31 (160 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.3)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: fontconfig
UpgradeStatus: Upgraded to disco on 2018-11-02 (157 days ago)