De esta discussão nos fóruns do Firefox:
[...] you can restore the bookmarks from a JSON backup in the bookmarkbackups folder.
- Bookmarks > Show All Bookmarks > Import & Backup > Restore
This will replace all current bookmarks and you lose bookmarks that were added since that backup was created. See:
In that case, you need to export the current bookmarks to an HTML file before doing the restore and import that HTML file after you have restored the JSON backup.
- Bookmark Deduplicator: https://addons.mozilla.org/firefox/addon/bookmark-deduplicator/