Make sure that the backup file with the bookmarks has the correct file extension: .html for a HTML backup and .json for a JSON backup.
You can check that via the right-click context menu of that file and open the Properties.
If you are not sure about the file type then you can open the file in Firefox via "File > Open File"
A JSON backup will show as one long text line without line breaks and a HTML backup as a web page with clickable links.

A JSON backup starts with: {"title":"","id":1,"dateAdded":
An HTML backup starts with: <!DOCTYPE NETSCAPE-Bookmark-file-1>

i know this dear {"title":"","id":1,"dateAdded":1311625795890000,"lastModified":1311626347227000,"type":"text/x-moz-place-container","root":"placesRoot",............................ but the problem was it was not at all recognising it as json file, may be some how automatically the extension changed to .file. but i solved it 1)open that file with firefox 2)copy the content from firefox 3)then open notepad 4) paste it 5) save file with anyname.json and change saveas type to all files 6) then import it through bookmark (show all bookmark->import)

thanks

Type "File" usually means that there is no file extension, so you need to add the .json file extension yourself.

This happens if you do not add a .json file extension while saving the bookmarks backup file.