diff --git a/mozilla/xpcom/typelib/xpidl/xpidl_header.c b/mozilla/xpcom/typelib/xpidl/xpidl_header.c
index a40e00729fb..2d433680ad6 100644
--- a/mozilla/xpcom/typelib/xpidl/xpidl_header.c
+++ b/mozilla/xpcom/typelib/xpidl/xpidl_header.c
@@ -68,21 +68,31 @@ pass_1(TreeState *state)
     return TRUE;
 }
 
-static gboolean
-write_classname_iid_define(FILE *file, const char *className)
+/*
+ * We expect something of the form 'prefIFoo', which becomes 'PREF_FOO'.
+ * Returns malloc()d string, please put little in its place.
+ */
+static char *
+classname_to_iid_def(const char *className)
 {
-    const char *iidName;
-    if (className[0] == 'n' && className[1] == 's') {
-        /* backcompat naming styles */
-        fputs("NS_", file);
-        iidName = className + 2;
-    } else {
-        iidName = className;
+    char *def, *copy;
+    gboolean donePrefix = FALSE;
+    size_t len = strlen(className) + 2;
+
+    copy = def = malloc(len);
+    if (!def)
+        return NULL;
+    while (*className) {
+        if (*className == 'I' && !donePrefix) {
+            *copy = '_';
+            donePrefix = TRUE;
+        } else {
+            *copy = toupper(*className);
+        }
+        copy++, className++;
     }
-    while (*iidName)
-        fputc(toupper(*iidName++), file);
-    fputs("_IID", file);
-    return TRUE;
+    *copy = '\0';
+    return def;
 }
 
 static gboolean
@@ -90,8 +100,13 @@ interface(TreeState *state)
 {
     IDL_tree iface = state->tree, iter;
     char *className = IDL_IDENT(IDL_INTERFACE(iface).ident).str;
+    gboolean ok = FALSE;
     const char *iid;
     const char *name_space;
+    char *classDef = classname_to_iid_def(className);
+
+    if (!classDef)
+        return FALSE;
 
     fprintf(state->file,   "\n/* starting interface:    %s */\n",
             className);
@@ -110,20 +125,19 @@ interface(TreeState *state)
         /* XXX use nsID parsing routines to validate? */
         if (strlen(iid) != 36)
             /* XXX report error */
-            return FALSE;
-        fprintf(state->file, "\n/* {%s} */\n#define ", iid);
-        if (!write_classname_iid_define(state->file, className))
-            return FALSE;
-        fprintf(state->file, "_STR \"%s\"\n#define ", iid);
-        if (!write_classname_iid_define(state->file, className))
-            return FALSE;
-        /* This is such a gross hack... */
-        fprintf(state->file, " \\\n  {0x%.8s, 0x%.4s, 0x%.4s, \\\n    "
-                "{ 0x%.2s, 0x%.2s, 0x%.2s, 0x%.2s, "
+            goto bad;
+        fprintf(state->file, "\n/* {%s} */\n"
+                "#define %s_IID_STR \"%s\"\n"
+                "#define %s_IID \\\n"
+                "  {0x%.8s, 0x%.4s, 0x%.4s, \\\n"
+                "    { 0x%.2s, 0x%.2s, 0x%.2s, 0x%.2s, "
                 "0x%.2s, 0x%.2s, 0x%.2s, 0x%.2s }}\n\n",
+                iid, classDef, iid, classDef,
+                /* This is such a gross hack... */
                 iid, iid + 9, iid + 14, iid + 19, iid + 21, iid + 24,
                 iid + 26, iid + 28, iid + 30, iid + 32, iid + 34);
     }
+
     fprintf(state->file, "class %s", className);
     if ((iter = IDL_INTERFACE(iface).inheritance_spec)) {
         fputs(" : ", state->file);
@@ -136,21 +150,22 @@ interface(TreeState *state)
     }
     fputs(" {\n"
           " public: \n", state->file);
-    if (iid) {
-        fputs("  NS_DEFINE_STATIC_IID_ACCESSOR(", state->file);
-        if (!write_classname_iid_define(state->file, className))
-            return FALSE;
-        fputs(")\n", state->file);
-    }
+
+    if (iid)
+        fprintf(state->file, "  NS_DEFINE_STATIC_IID_ACCESSOR(%s_IID)\n",
+                classDef);
 
     state->tree = IDL_INTERFACE(iface).body;
 
     if (state->tree && !xpidl_process_node(state))
-        return FALSE;
+        goto bad;
 
     fputs("};\n", state->file);
 
-    return TRUE;
+    ok = TRUE;
+ bad:
+    free(classDef);
+    return ok;
 }
 
 static gboolean