A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox

• http://kb.mozillazine.org/Lost_bookmarks

For future prevention see:

• http://kb.mozillazine.org/Backing_up_and_restoring_bookmarks_-_Firefox
• http://kb.mozillazine.org/Profile_backup

Thanks cor-el, but I was unable to resolve my problem with the information you provided. I have my bookmarks backed-up but, when I try to restore them, I get a message that says 'Unable to process the backup file.'

I also tried to save to bookmarks, and that won't work either.

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>

Add quotes ("") around the name to rename the file to the correct file extension if the current file extension is hidden or if it is otherwise not working.