Are you using Sync?

Or have you imported a bookmarks.html file from another Firefox installation?

Yes I have imported a bookmarks.html file

Importing a bookmarks.html file will append existing bookmarks and folders. You can delete the extra Bookmarks Toolbar folders after moving the bookmarks elsewhere, the extra Unsorted Bookmarks folder after moving the contents elsewhere, one of the extra Recently Bookmarked folders, and the extra Recent Tags folder. Those "Recent" folders don't have the actual bookmarks in them; just like "recent history" of new bookmarks and tags - the real bookmarks are saved elsewhere.

Thanks!!..Now i understand