There were some changes to the Places database format around Firefox 56 that makes it incompatible with older versions of Firefox.

You might need to try the steps in this article -- trigger Firefox to generate a new database:

Fix "The bookmarks and history system will not be functional" error message

Also, why Firefox 50? If you want to use an older version, please use the "Extended Support Release" (ESR) version: Switch to Firefox Extended Support Release (ESR) for personal use.

On the download page, above the list of different languages, there is a selector on the right side to choose Firefox 60 or Firefox 52.

I did not see your earlier question https://support.mozilla.org/questions/1221981 but I would have the same suggestion.

If "Verify Integrity" on the "Help -> Troubleshooting Information" (about:support) page cannot repair places.sqlite then remove all places.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created
• http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

See also:

• http://kb.mozillazine.org/Locked_or_damaged_places.sqlite

Огромное спасибо! Удаление файла places.sqlite решило проблему с закладками! Ура!!!

Thank you so much! Removing the places.sqlite file solved the problem with bookmarks! Hooray!!!