Disables Font Loading API for Firefox.

src/display/font_loader.js

@@ -118,6 +118,10 @@ FontLoader.prototype = {
         warn('Failed to load font "' + nativeFontFace.family + '": ' + e);
       });
     };
+    // Firefox Font Loading API does not work with mozPrintCallback --
+    // disabling it in this case.
+    var isFontLoadingAPISupported = FontLoader.isFontLoadingAPISupported &&
+                                    !FontLoader.isSyncFontLoadingSupported;
     for (var i = 0, ii = fonts.length; i < ii; i++) {
       var font = fonts[i];
 
@@ -128,7 +132,7 @@ FontLoader.prototype = {
       }
       font.attached = true;
 
-      if (FontLoader.isFontLoadingAPISupported) {
+      if (isFontLoadingAPISupported) {
         var nativeFontFace = font.createNativeFontFace();
         if (nativeFontFace) {
           this.addNativeFontFace(nativeFontFace);
@@ -145,7 +149,7 @@ FontLoader.prototype = {
     }
 
     var request = this.queueLoadingCallback(callback);
-    if (FontLoader.isFontLoadingAPISupported) {
+    if (isFontLoadingAPISupported) {
       Promise.all(fontLoadPromises).then(function() {
         request.complete();
       });