After I upgraded my firefox to version 55, I found that old option:

gfx.font_rendering.fontconfig.fontlist.enabled workaround 

no longer existed, So without that option I can not display emoji in firefox correctly.

选区_117.png

There is a way to fix this problem, Just try to make font emoji-color-one to be used before your oter fonts.

sudo gedit /etc/fonts/conf.avail/69-language-selector-zh-cn.conf

Try to add:

<string>EmojiOne Color</string>

After your Bitstream font family. Just like this.

选区_119.png

And you can check if it works.

$ fc-match -s sans | head -n3
Vera.ttf: "Bitstream Vera Sans" "Roman"
EmojiOneColor-SVGinOT.ttf: "EmojiOne Color" "Regular"
wqy-microhei.ttc: "文泉驿微米黑" "Regular"

And finally……

选区_118.png

Here is the way to install emojione-color-font.