First, let's check the bookmarks file for corruption.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

When done, copy and post the results here.

> Task: checkIntegrity + The places.sqlite database is sane + The favicons.sqlite database is sane > Task: invalidateCaches + The caches have been invalidated > Task: checkCoherence + The database is coherent > Task: expire + Database cleaned up > Task: originFrecencyStats + Recalculated origin frecency stats > Task: vacuum + Initial database size is 15360KiB + The database has been vacuumed + Final database size is 15360KiB > Task: stats + Places.sqlite size is 15360KiB + Favicons.sqlite size is 14784KiB + pragma_user_version is 52 + pragma_page_size is 32768 + pragma_cache_size is -2048 + pragma_journal_mode is wal + pragma_synchronous is 1 + History can store a maximum of 133069 unique pages + Table moz_places has 23394 records + Table moz_historyvisits has 33506 records + Table moz_inputhistory has 31 records + Table moz_hosts has 0 records + Table moz_bookmarks has 648 records + Table moz_keywords has 0 records + Table sqlite_sequence has 0 records + Table moz_anno_attributes has 6 records + Table moz_annos has 164 records + Table moz_items_annos has 9 records + Table sqlite_stat1 has 18 records + Table moz_bookmarks_deleted has 0 records + Table moz_meta has 7 records + Table moz_origins has 3213 records + Index sqlite_autoindex_moz_inputhistory_1 + Index sqlite_autoindex_moz_hosts_1 + Index sqlite_autoindex_moz_keywords_1 + Index sqlite_autoindex_moz_anno_attributes_1 + Index sqlite_autoindex_moz_bookmarks_deleted_1 + Index sqlite_autoindex_moz_origins_1 + Index moz_places_hostindex + Index moz_places_visitcount + Index moz_places_frecencyindex + Index moz_places_lastvisitdateindex + Index moz_historyvisits_placedateindex + Index moz_historyvisits_fromindex + Index moz_historyvisits_dateindex + Index moz_bookmarks_itemindex + Index moz_bookmarks_parentindex + Index moz_bookmarks_itemlastmodifiedindex + Index moz_places_url_hashindex + Index moz_places_guid_uniqueindex + Index moz_bookmarks_guid_uniqueindex + Index moz_keywords_placepostdata_uniqueindex + Index moz_annos_placeattributeindex + Index moz_items_annos_itemattributeindex + Index moz_bookmarks_dateaddedindex + Index moz_places_originidindex > Task: _refreshUI

The database looks okay. Have you opened the Bookmarks Manager and looked around?

Restore Bookmark Backups

Firefox automatically makes backups of the bookmarks. These are stored in the bookmarkbackups folder.

Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B.

Once the window is open, at the top of the page, press the button labeled Import and Backup. Select Restore. From the list, pick the file you want and click it.

The backups are stored in a jsonlz4 file named something like this; bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4

In this example; 2016-06-29 = date created, 1424 = number of entries.

Note: Look for the file from the day before the data disappeared.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks