--- a/browser/components/preferences/in-content-old/tests/browser_basic_rebuild_fonts_test.js
+++ b/browser/components/preferences/in-content-old/tests/browser_basic_rebuild_fonts_test.js
@@ -22,55 +22,88 @@ add_task(function*() {
 
   doc.getElementById("advancedFonts").click();
   let win = yield promiseLoadSubDialog("chrome://browser/content/preferences/fonts.xul");
   doc = win.document;
 
   // Simulate a dumb font backend.
   win.FontBuilder._enumerator = {
     _list: ["MockedFont1", "MockedFont2", "MockedFont3"],
+    _defaultFont: null,
     EnumerateFonts(lang, type, list) {
       return this._list;
     },
     EnumerateAllFonts() {
       return this._list;
     },
-    getDefaultFont() { return null; },
+    getDefaultFont() { return this._defaultFont; },
     getStandardFamilyName(name) { return name; },
   };
   win.FontBuilder._allFonts = null;
   win.FontBuilder._langGroupSupported = false;
 
   let langGroupElement = doc.getElementById("font.language.group");
   let selectLangsField = doc.getElementById("selectLangs");
   let serifField = doc.getElementById("serif");
   let armenian = "x-armn";
   let western = "x-western";
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
   is(serifField.value, "", "Font family should not be set.");
 
+  let armenianSerifElement = doc.getElementById("font.name.serif.x-armn");
+
   langGroupElement.value = western;
   selectLangsField.value = western;
 
   // Simulate a font backend supporting language-specific enumeration.
   // NB: FontBuilder has cached the return value from EnumerateAllFonts(),
   // so _allFonts will always have 3 elements regardless of subsequent
   // _list changes.
   win.FontBuilder._enumerator._list = ["MockedFont2"];
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
-  is(serifField.value, "MockedFont2", "Font family should be set.");
+  is(serifField.value, "", "Font family should still be empty for indicating using 'default' font.");
 
   langGroupElement.value = western;
   selectLangsField.value = western;
 
   // Simulate a system that has no fonts for the specified language.
   win.FontBuilder._enumerator._list = [];
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
   is(serifField.value, "", "Font family should not be set.");
 
+  // Setting default font to "MockedFont3".  Then, when serifField.value is
+  // empty, it should indicate using "MockedFont3" but it shouldn't be saved
+  // to "MockedFont3" in the pref.  It should be resolved at runtime.
+  win.FontBuilder._enumerator._list = ["MockedFont1", "MockedFont2", "MockedFont3"];
+  win.FontBuilder._enumerator._defaultFont = "MockedFont3";
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "", "Font family should be empty even if there is a default font.");
+
+  armenianSerifElement.value = "MockedFont2";
+  serifField.value = "MockedFont2";
+  is(serifField.value, "MockedFont2", "Font family should be \"MockedFont2\" for now.");
+
+  langGroupElement.value = western;
+  selectLangsField.value = western;
+  is(serifField.value, "", "Font family of other language should not be set.");
+
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "MockedFont2", "Font family should not be changed even after switching the language.");
+
+  // If MochedFont2 is removed from the system, the value should be treated
+  // as empty (i.e., 'default' font) after rebuilding the font list.
+  win.FontBuilder._enumerator._list = ["MockedFont1", "MockedFont3"];
+  win.FontBuilder._enumerator._allFonts = ["MockedFont1", "MockedFont3"];
+  serifField.removeAllItems(); // This will cause rebuilding the font list from available fonts.
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "", "Font family should become empty due to the font uninstalled.");
+
   gBrowser.removeCurrentTab();
 });

--- a/browser/components/preferences/in-content/tests/browser_basic_rebuild_fonts_test.js
+++ b/browser/components/preferences/in-content/tests/browser_basic_rebuild_fonts_test.js
@@ -22,55 +22,88 @@ add_task(function*() {
 
   doc.getElementById("advancedFonts").click();
   let win = yield promiseLoadSubDialog("chrome://browser/content/preferences/fonts.xul");
   doc = win.document;
 
   // Simulate a dumb font backend.
   win.FontBuilder._enumerator = {
     _list: ["MockedFont1", "MockedFont2", "MockedFont3"],
+    _defaultFont: null,
     EnumerateFonts(lang, type, list) {
       return this._list;
     },
     EnumerateAllFonts() {
       return this._list;
     },
-    getDefaultFont() { return null; },
+    getDefaultFont() { return this._defaultFont; },
     getStandardFamilyName(name) { return name; },
   };
   win.FontBuilder._allFonts = null;
   win.FontBuilder._langGroupSupported = false;
 
   let langGroupElement = doc.getElementById("font.language.group");
   let selectLangsField = doc.getElementById("selectLangs");
   let serifField = doc.getElementById("serif");
   let armenian = "x-armn";
   let western = "x-western";
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
   is(serifField.value, "", "Font family should not be set.");
 
+  let armenianSerifElement = doc.getElementById("font.name.serif.x-armn");
+
   langGroupElement.value = western;
   selectLangsField.value = western;
 
   // Simulate a font backend supporting language-specific enumeration.
   // NB: FontBuilder has cached the return value from EnumerateAllFonts(),
   // so _allFonts will always have 3 elements regardless of subsequent
   // _list changes.
   win.FontBuilder._enumerator._list = ["MockedFont2"];
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
-  is(serifField.value, "MockedFont2", "Font family should be set.");
+  is(serifField.value, "", "Font family should still be empty for indicating using 'default' font.");
 
   langGroupElement.value = western;
   selectLangsField.value = western;
 
   // Simulate a system that has no fonts for the specified language.
   win.FontBuilder._enumerator._list = [];
 
   langGroupElement.value = armenian;
   selectLangsField.value = armenian;
   is(serifField.value, "", "Font family should not be set.");
 
+  // Setting default font to "MockedFont3".  Then, when serifField.value is
+  // empty, it should indicate using "MockedFont3" but it shouldn't be saved
+  // to "MockedFont3" in the pref.  It should be resolved at runtime.
+  win.FontBuilder._enumerator._list = ["MockedFont1", "MockedFont2", "MockedFont3"];
+  win.FontBuilder._enumerator._defaultFont = "MockedFont3";
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "", "Font family should be empty even if there is a default font.");
+
+  armenianSerifElement.value = "MockedFont2";
+  serifField.value = "MockedFont2";
+  is(serifField.value, "MockedFont2", "Font family should be \"MockedFont2\" for now.");
+
+  langGroupElement.value = western;
+  selectLangsField.value = western;
+  is(serifField.value, "", "Font family of other language should not be set.");
+
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "MockedFont2", "Font family should not be changed even after switching the language.");
+
+  // If MochedFont2 is removed from the system, the value should be treated
+  // as empty (i.e., 'default' font) after rebuilding the font list.
+  win.FontBuilder._enumerator._list = ["MockedFont1", "MockedFont3"];
+  win.FontBuilder._enumerator._allFonts = ["MockedFont1", "MockedFont3"];
+  serifField.removeAllItems(); // This will cause rebuilding the font list from available fonts.
+  langGroupElement.value = armenian;
+  selectLangsField.value = armenian;
+  is(serifField.value, "", "Font family should become empty due to the font uninstalled.");
+
   gBrowser.removeCurrentTab();
 });

--- a/toolkit/mozapps/preferences/fontbuilder.js
+++ b/toolkit/mozapps/preferences/fontbuilder.js
@@ -95,28 +95,14 @@ var FontBuilder = {
     if (preference.value) {
       let fontItems = aElement.getElementsByAttribute("value", preference.value);
 
       // There is a setting that actually is in the list. Respect it.
       if (fontItems.length)
         return undefined;
     }
 
-    // The first item will be a reasonable choice only if the font backend
-    // supports language-specific enumaration.
-    let defaultValue = this._langGroupSupported ?
-                       aElement.firstChild.firstChild.getAttribute("value") : "";
-    let fontNameList = preference.name.replace(".name.", ".name-list.");
-    let prefFontNameList = document.getElementById(fontNameList);
-    if (!prefFontNameList || !prefFontNameList.value)
-      return defaultValue;
-
-    let fontNames = prefFontNameList.value.split(",");
-
-    for (let i = 0; i < fontNames.length; ++i) {
-      let fontName = this.enumerator.getStandardFamilyName(fontNames[i].trim());
-      let fontItems = aElement.getElementsByAttribute("value", fontName);
-      if (fontItems.length)
-        return fontItems[0].getAttribute("value");
-    }
-    return defaultValue;
+    // Otherwise, use "default" font of current system which is computed
+    // with "font.name-list.*".  If "font.name.*" is empty string, it means
+    // "default".  So, return empty string in this case.
+    return "";
   }
 };