The best way t replace them is find the bookmarkbackups folder whci daily makes a backup of your bookmarks
To see this you need to go in to folder options and check 'show hidden folders'
You can then cope the latest backup -paste it on your desktop - then import from file
More here
http://kb.mozillazine.org/Bookmarks
Remember Firefox allows you to backup your bookmarks whenever you want just go to Organise bookmarks>file>export then you can save a copy to wherever you like - great to transfer to another PC you use