Set font.name-list.emoji for some platforms.

- Twemoji Mozilla for Windows. Can be prefixed with Segoe UI emoji for native Win8+ emoji, but reduces uniformity.
- Apple Color Emoji for OSX
- Twemoji Mozilla for GTK (Linux). fontconfig doesn't support emoji as family name.
- Noto Color Emoji for Android

modules/libpref/init/all.js

@@ -3171,6 +3171,10 @@ pref("ui.mouse.radius.inputSource.touchOnly", true);
 
 #ifdef XP_WIN
 
+// Be as uniform as possible, use Twemoji everywhere. 
+// Optional: prefix with `Segoe UI Emoji` to use Win8+ Segoe UI font emoji where available.
+pref("font.name-list.emoji", "Twemoji Mozilla");
+
 pref("font.name.serif.ar", "Times New Roman");
 pref("font.name.sans-serif.ar", "Segoe UI");
 pref("font.name-list.sans-serif.ar", "Segoe UI, Tahoma, Arial");
@@ -3604,6 +3608,8 @@ pref("ui.osk.debug.keyboardDisplayReason", "");
 pref("browser.drag_out_of_frame_style", 1);
 pref("ui.key.saveLink.shift", false); // true = shift, false = meta
 
+pref("font.name-list.emoji", "Apple Color Emoji");
+
 // default fonts (in UTF8 and using canonical names)
 // to determine canonical font names, use a debug build and
 // enable NSPR logging for module fontInfoLog:5
@@ -4026,6 +4032,10 @@ pref("print.print_extra_margin", 0); // twips
 
 // font names
 
+// fontconfig doesn't support emoji yet
+// https://lists.freedesktop.org/archives/fontconfig/2016-October/005842.html
+pref("font.name-list.emoji", "Twemoji Mozilla");
+
 pref("font.name.serif.ar", "serif");
 pref("font.name.sans-serif.ar", "sans-serif");
 pref("font.name.monospace.ar", "monospace");
@@ -4270,6 +4280,8 @@ pref("font.name.monospace.x-math", "Fira Mono");
 #elif defined(ANDROID)
 // We use the bundled fonts for Firefox for Android
 
+pref("font.name-list.emoji", "Noto Color Emoji");
+
 // ar
 
 pref("font.name.serif.el", "Droid Serif"); // not Charis SIL Compact, only has a few Greek chars