Make sure that the JSON backup has a .json file extension.
You can verify that via the Properties in the right-click context menu of the file.

yes, correctly, and i'm sure the bookmarks has been exported as a JSON file. my problem is i found only HTML bookmarks can be imported in the latest version FIREFOX 14.0.1. so i want to know how to import JSON back-up in the new version FIREFOX???? thanks

Firefox doesn't append a file extension by default when you backup the bookmarks to a JSON file, so if you didn't add it yourself when saving then the file doesn't have a .json file extension.
Firefox will only restore a JSON backup if the file has a .json file extension.

Did you verify in the properties that the file type is JSON?