diff --git a/xpinstall/src/nsInstall.cpp b/xpinstall/src/nsInstall.cpp
index 36e92e37a7dc..ea1256767678 100644
--- a/xpinstall/src/nsInstall.cpp
+++ b/xpinstall/src/nsInstall.cpp
@@ -751,10 +751,6 @@ nsInstall::GetComponentFolder(const nsString& aComponentName, const nsString& aS
         if(aSubdirectory != "")
         {
           nsfsDir += aSubdirectory;
-          if(!nsfsDir.Exists())
-          {
-              nsfsDir.CreateDirectory();
-          }
         }
         *aFolder  = new nsString(nsfsDir.GetNativePathCString());
     }
diff --git a/xpinstall/src/nsInstall.h b/xpinstall/src/nsInstall.h
index 90e2f0255d16..5a143463e5f3 100644
--- a/xpinstall/src/nsInstall.h
+++ b/xpinstall/src/nsInstall.h
@@ -122,6 +122,7 @@ class nsInstall
             UNINSTALL_FAILED            = -223,
             PACKAGE_FOLDER_NOT_SET      = -224,
             EXTRACTION_FAILED           = -225,
+            FILENAME_ALREADY_USED       = -226,
             GESTALT_UNKNOWN_ERR         = -5550,
             GESTALT_INVALID_ARGUMENT    = -5551,
             
diff --git a/xpinstall/src/nsInstallFile.cpp b/xpinstall/src/nsInstallFile.cpp
index a5e1433567af..aadeab62294e 100644
--- a/xpinstall/src/nsInstallFile.cpp
+++ b/xpinstall/src/nsInstallFile.cpp
@@ -107,14 +107,36 @@ nsInstallFile::nsInstallFile(nsInstall* inInstall,
 
     delete [] qualifiedRegNameString;
 
-    
-     mFinalFile = new nsFileSpec(folderSpec);
+    mFinalFile = new nsFileSpec(folderSpec);
+
+    if ( mFinalFile->Exists() )
+    {
+        // is there a file with the same name as the proposed folder?
+        if ( mFinalFile->IsFile() ) 
+        {
+            *error = nsInstall::FILENAME_ALREADY_USED;
+            return;
+        }
+        // else this directory already exists, so do nothing
+    }
+    else
+    {
+        /* the nsFileSpecMac.cpp operator += requires "this" (the nsFileSpec)
+         * to be an existing dir
+         */
+        int dirPermissions = 755; // std default for UNIX, ignored otherwise
+        mFinalFile->CreateDir(dirPermissions);
+    }
+
     *mFinalFile += inPartialPath;
     
     mReplaceFile = mFinalFile->Exists();
     
     if (mReplaceFile == PR_FALSE)
     {
+       /* although it appears that we are creating the dir _again_ it is necessary
+        * when inPartialPath has arbitrary levels of nested dirs before the leaf
+        */
         nsFileSpec parent;
         mFinalFile->GetParent(parent);
         nsFileSpec makeDirs(parent.GetCString(), PR_TRUE);
diff --git a/xpinstall/src/nsInstallFolder.cpp b/xpinstall/src/nsInstallFolder.cpp
index c456a3ec4d79..76eed682a411 100644
--- a/xpinstall/src/nsInstallFolder.cpp
+++ b/xpinstall/src/nsInstallFolder.cpp
@@ -106,7 +106,10 @@ nsInstallFolder::nsInstallFolder(const nsString& aFolderID, const nsString& aRel
         tempString += aRelativePath;
         mFileSpec = new nsFileSpec(tempString);
 
-        // make sure that the directory is created.
+        // This paranoia makes no sense since dirCheck.IsDirectory() && dirCheck.Exists() !
+        //      vvvv
+        // make sure that the directory is created. 
+        //      ^^^^
         nsFileSpec(mFileSpec->GetCString(), PR_TRUE);
     }
     else
@@ -273,7 +276,7 @@ nsInstallFolder::SetDirectoryPath(const nsString& aFolderID, const nsString& aRe
                 mFileSpec = nsnull;
 			   return;
 		}
-#ifndef XP_MAC
+		
         if (aRelativePath.Length() > 0)
         {
             nsString tempPath(aRelativePath);
@@ -283,9 +286,6 @@ nsInstallFolder::SetDirectoryPath(const nsString& aFolderID, const nsString& aRe
 
             *mFileSpec += tempPath;
         }
-#endif
-            // make sure that the directory is created.
-        nsFileSpec(mFileSpec->GetCString(), PR_TRUE);
     }
 }
 
diff --git a/xpinstall/src/nsJSInstall.cpp b/xpinstall/src/nsJSInstall.cpp
index df4d431f36db..b04dd18cc990 100644
--- a/xpinstall/src/nsJSInstall.cpp
+++ b/xpinstall/src/nsJSInstall.cpp
@@ -2344,6 +2344,9 @@ static JSConstDoubleSpec install_constants[] =
     { nsInstall::PATCH_BAD_CHECKSUM_TARGET,  "PATCH_BAD_CHECKSUM_TARGET"    },
     { nsInstall::PATCH_BAD_CHECKSUM_RESULT,  "PATCH_BAD_CHECKSUM_RESULT"    },
     { nsInstall::UNINSTALL_FAILED,           "UNINSTALL_FAILED"             },
+    { nsInstall::PACKAGE_FOLDER_NOT_SET,     "PACKAGE_FOLDER_NOT_SET"       },
+    { nsInstall::EXTRACTION_FAILED,          "EXTRACTION_FAILED"            },
+    { nsInstall::FILENAME_ALREADY_USED,      "FILENAME_ALREADY_USED"        },
     { nsInstall::GESTALT_UNKNOWN_ERR,        "GESTALT_UNKNOWN_ERR"          },
     { nsInstall::GESTALT_INVALID_ARGUMENT,   "GESTALT_INVALID_ARGUMENT"     },
     { nsInstall::SUCCESS,                    "SUCCESS"                      },