After I upgraded my firefox to version 55, I found that old option:
no longer existed, So without that option I can not display emoji in firefox correctly.
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:
After your Bitstream font family. Just like this.
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"
Here is the way to install emojione-color-font.