You can check for problems with the <b>places.sqlite</b> database (bookmarks and history) in the Firefox profile folder. *https://addons.mozilla.org/firefox/addon/places-maintenance/ See also: *http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox *https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file You can check the backups in the bookmarkbackups folder to see if you can locate a backup this isn't infected yet. *Bookmarks -> Show All Bookmarks -> Import & Backup > Restore The name of a JSON bookmarks backup file includes a total item count (folders and separators included) and an hash value to prevent saving the same backup more than once. *bookmarks-YYYY-MM-DD_<item count>_<hash>.jsonlz4. If you use Sync then best is to disconnect Sync while performing maintenance/repair efforts to files in the Firefox profile folder. Best is to (re)connect one device at the time and wait for the initial Sync to get finished before connecting another device once you have fixed the bookmarks.