Followup to Bug 476106 (Win32 only) - Unable to get directory path for directories immediately under some symlinks. r=jmathies

browser/installer/windows/nsis/uninstaller.nsi

@@ -559,7 +559,7 @@ Function .onInit
 FunctionEnd
 
 Function un.onInit
-  GetFullPathName $INSTDIR "$INSTDIR\.."
+  ${un.GetParent} "$INSTDIR" $INSTDIR
   ${un.GetLongPath} "$INSTDIR" $INSTDIR
   ${Unless} ${FileExists} "$INSTDIR\${FileMainEXE}"
     Abort