diff --git a/gfx/thebes/gfxFontUtils.cpp b/gfx/thebes/gfxFontUtils.cpp
index 062ab9667afe..22e659327900 100644
--- a/gfx/thebes/gfxFontUtils.cpp
+++ b/gfx/thebes/gfxFontUtils.cpp
@@ -379,9 +379,8 @@ gfxFontUtils::ReadCMAPTableFormat14(const uint8_t *aBuf, uint32_t aLength,
     return NS_OK;
 }
 
-// Windows requires fonts to have a format-4 cmap with a Microsoft ID (3).  On the Mac, fonts either have
-// a format-4 cmap with Microsoft platform/encoding id or they have one with a platformID == Unicode (0)
-// For fonts with two format-4 tables, the first one (Unicode platform) is preferred on the Mac.
+// For fonts with two format-4 tables, the first one (Unicode platform) is preferred on the Mac;
+// on other platforms we allow the Microsoft-platform subtable to replace it.
 
 #if defined(XP_MACOSX)
     #define acceptableFormat4(p,e,k) (((p) == PLATFORM_ID_MICROSOFT && (e) == EncodingIDMicrosoft && !(k)) || \
@@ -392,7 +391,8 @@ gfxFontUtils::ReadCMAPTableFormat14(const uint8_t *aBuf, uint32_t aLength,
          ((p) == PLATFORM_ID_UNICODE   && \
           ((e) != EncodingIDUVSForUnicodePlatform)))
 #else
-    #define acceptableFormat4(p,e,k) ((p) == PLATFORM_ID_MICROSOFT && (e) == EncodingIDMicrosoft)
+    #define acceptableFormat4(p,e,k) (((p) == PLATFORM_ID_MICROSOFT && (e) == EncodingIDMicrosoft) || \
+                                      ((p) == PLATFORM_ID_UNICODE))
 
     #define acceptableUCS4Encoding(p, e, k) \
         ((p) == PLATFORM_ID_MICROSOFT && (e) == EncodingIDUCS4ForMicrosoftPlatform)