diff --git a/xulrunner/stub/nsXULStub.cpp b/xulrunner/stub/nsXULStub.cpp
--- a/xulrunner/stub/nsXULStub.cpp
+++ b/xulrunner/stub/nsXULStub.cpp
@@ -109,16 +109,54 @@ static void Output(PRBool isError, const
   MessageBoxW(NULL, wide_msg, L"XULRunner", flags);
 #else
   vfprintf(stderr, fmt, ap);
 #endif
 
   va_end(ap);
 }
 
+/**
+ * Return true if |arg| matches the given argument name.
+ */
+static PRBool IsArg(const char* arg, const char* s)
+{
+  if (*arg == '-')
+  {
+    if (*++arg == '-')
+      ++arg;
+    return !strcasecmp(arg, s);
+  }
+
+#if defined(XP_WIN) || defined(XP_OS2)
+  if (*arg == '/')
+    return !strcasecmp(++arg, s);
+#endif
+
+  return PR_FALSE;
+}
+
+static nsresult SetEnvironmentVariable(const char* aEnvVar, const char* aValue)
+{
+#if defined(XP_MACOSX) || defined(XP_UNIX)
+  if (!setenv(aEnvVar, aValue, 1))
+    return NS_OK;
+#elif defined(WINCE)
+  static char kAppEnv[MAXPATHLEN];
+  _snwprintf(kAppEnv, MAXPATHLEN, L"%s=%s", aEnvVar, aValue);
+  if (!putenv(kAppEnv))
+    return NS_OK;
+#elif defined(XP_WIN)
+  if(::SetEnvironmentVariableA(aEnvVar, aValue))
+    return NS_OK;
+#endif
+
+  return NS_ERROR_FAILURE;
+}
+
 class AutoAppData
 {
 public:
   AutoAppData(nsILocalFile* aINIFile) : mAppData(nsnull) {
     nsresult rv = XRE_CreateAppData(aINIFile, &mAppData);
     if (NS_FAILED(rv))
       mAppData = nsnull;
   }
@@ -144,60 +182,99 @@ main(int argc, char **argv)
   nsresult rv;
   char *lastSlash;
 
   char iniPath[MAXPATHLEN];
   char tmpPath[MAXPATHLEN];
   char greDir[MAXPATHLEN];
   PRBool greFound = PR_FALSE;
 
+  const char *appDataFile;
+#if defined(XP_WIN)
+  ::GetEnvironmentVariableA("XUL_APP_FILE", appDataFile, MAXPATHLEN);
+#else
+  appDataFile = getenv("XUL_APP_FILE");
+#endif
+  if (!appDataFile || !*appDataFile) 
+    if (argc > 1 && IsArg(argv[1], "app")) {
+      if (argc == 2) {
+        Output(PR_FALSE, "specify APP-FILE (optional)\n");
+        return 1;
+      }
+      argv[1] = argv[0];
+      ++argv;
+      --argc;
+
+      appDataFile = argv[1];
+      argv[1] = argv[0];
+      ++argv;
+      --argc;
+
+      const char kEnvVar[] = "XUL_APP_FILE";
+      rv = SetEnvironmentVariable(kEnvVar, appDataFile);
+      if (NS_FAILED(rv))
+        Output(PR_FALSE, "Couldn't set %s.\n", kEnvVar);
+    }
+
 #if defined(XP_MACOSX)
   CFBundleRef appBundle = CFBundleGetMainBundle();
   if (!appBundle)
     return 1;
 
   CFURLRef resourcesURL = CFBundleCopyResourcesDirectoryURL(appBundle);
   if (!resourcesURL)
     return 1;
 
   CFURLRef absResourcesURL = CFURLCopyAbsoluteURL(resourcesURL);
   CFRelease(resourcesURL);
   if (!absResourcesURL)
     return 1;
 
+  CFStringRef appDataFileStr;
+  appDataFileStr = (appDataFile && *appDataFile) ?
+    CFStringCreateWithCString(NULL, appDataFile, kCFStringEncodingUTF8) :
+    CFSTR("application.ini");
+
   CFURLRef iniFileURL =
     CFURLCreateCopyAppendingPathComponent(kCFAllocatorDefault,
                                           absResourcesURL,
-                                          CFSTR("application.ini"),
+                                          appDataFileStr,
                                           false);
   CFRelease(absResourcesURL);
   if (!iniFileURL)
     return 1;
 
   CFStringRef iniPathStr =
     CFURLCopyFileSystemPath(iniFileURL, kCFURLPOSIXPathStyle);
   CFRelease(iniFileURL);
+  CFRelease(appDataFileStr);
   if (!iniPathStr)
     return 1;
 
   CFStringGetCString(iniPathStr, iniPath, sizeof(iniPath),
                      kCFStringEncodingUTF8);
   CFRelease(iniPathStr);
 
 #else
 
-#ifdef XP_WIN
-  wchar_t wide_path[MAX_PATH];
-  if (!::GetModuleFileNameW(NULL, wide_path, MAX_PATH))
-    return 1;
-
-  WideCharToMultiByte(CP_ACP, 0, wide_path,-1,
-		      iniPath, MAX_PATH, NULL, NULL);
-  
-
+#if defined (XP_WIN)
+  if (!(appDataFile && *appDataFile)) {
+    wchar_t wide_path[MAX_PATH];
+    if (!::GetModuleFileNameW(NULL, wide_path, MAX_PATH))
+      return 1;  
+    
+    WideCharToMultiByte(CP_ACP, 0, wide_path,-1,
+                        iniPath, MAX_PATH, NULL, NULL);
+  } else {
+    wchar_t wAppDataFile[MAX_PATH];
+    wchar_t wIniPath[MAX_PATH];
+    MultiByteToWideChar(CP_ACP, 0, appDataFile, -1, wAppDataFile, MAX_PATH);
+    _wfullpath(wIniPath, wAppDataFile, MAX_PATH);
+    WideCharToMultiByte(CP_ACP, 0, wIniPath, -1, iniPath, MAX_PATH, 0, 0);
+  }
 #elif defined(XP_OS2)
    PPIB ppib;
    PTIB ptib;
 
    DosGetInfoBlocks(&ptib, &ppib);
    DosQueryModuleName(ppib->pib_hmte, sizeof(iniPath), iniPath);
 
 #elif defined(XP_BEOS)
@@ -218,38 +295,40 @@ main(int argc, char **argv)
   //
   // 1) use realpath() on argv[0], which works unless we're loaded from the
   //    PATH
   // 2) manually walk through the PATH and look for ourself
   // 3) give up
 
   struct stat fileStat;
 
-  if (!realpath(argv[0], iniPath) || stat(iniPath, &fileStat)) {
-    const char *path = getenv("PATH");
-    if (!path)
-      return 1;
+  if (!realpath(appDataFile, iniPath) || stat(iniPath, &fileStat)) {
+    if (!realpath(argv[0], iniPath) || stat(iniPath, &fileStat)) {
+      const char *path = getenv("PATH");
+      if (!path)
+        return 1;
 
     char *pathdup = strdup(path);
     if (!pathdup)
       return 1;
 
-    PRBool found = PR_FALSE;
-    char *token = strtok(pathdup, ":");
-    while (token) {
-      sprintf(tmpPath, "%s/%s", token, argv[0]);
-      if (realpath(tmpPath, iniPath) && stat(iniPath, &fileStat) == 0) {
-        found = PR_TRUE;
-        break;
+      PRBool found = PR_FALSE;
+      char *token = strtok(pathdup, ":");
+      while (token) {
+        sprintf(tmpPath, "%s/%s", token, argv[0]);
+        if (realpath(tmpPath, iniPath) && stat(iniPath, &fileStat) == 0) {
+          found = PR_TRUE;
+          break;
+        }
+        token = strtok(NULL, ":");
       }
-      token = strtok(NULL, ":");
+      free (pathdup);
+      if (!found)
+        return 1;
     }
-    free (pathdup);
-    if (!found)
-      return 1;
   }
 #endif
 
   lastSlash = strrchr(iniPath, PATH_SEPARATOR_CHAR);
   if (!lastSlash)
     return 1;
 
   *(++lastSlash) = '\0';