Have you saved any URLs to ma.gnolia since your last backup? It may be present in your Firefox history! I was able to recover all my un-backed-up ma.gnolia links (though not their descriptions or tags, of course) by this method:

  1. Show all history
  2. Filter by ma.gnolia.com/bookmarklet/
  3. Select all, copy
  4. Create new bookmark folder called `magback` (the name is important!)
  5. Paste into `magback` folder
  6. Show "Visit Date" column, sort by that
  7. Delete everything that was before your last backup
  8. Export as HTML, save as `extract.html`
  9. Use a text editor to add the following to the end of the file: 
    <script src="jquery.js" type="text/javascript"></script><script src="restore.js" type="text/javascript"></script>
  10. Save these files in the same folder where you put `extract.html`: jquery.js, restore.js
  11. View the HTML page in Firefox
  12. A download box for an file will be generated. Accept the file, and name it something like `magnolia-import.html`.
  13. Import the new file into Firefox's bookmarks.
  14. Your recovered ma.gnolia bookmarks will be in a new folder called "Recovered from ma.gnolia".
  15. You can now delete the `magback` folder.