For what you need to restore Firefox bookmark? Maybe you want to restore it when you want to reinstall your Firefox or just when you accidentally delete it or when it was corrupted. If you just have a few bookmarks maybe it doesn’t problem for you but how if you have many bookmark sites? It will take your time to remember some of them and surely will impact your job.
Firefox provides automatic backup Firefox bookmark for last 11 day and manual backup. To backup manually just click Firefox button at top left Firefox and dig Bookmarks and click Show All Bookmark or just with press CTRL + SHIFT + b from your keyboard as shortcut to open bookmark manager and just click Import and Backup and click Backup.
Automatically it will suggest file name base on date created and provide JSON file as extension and save it to your computer.
When you have a problem with bookmark you can easily restore it. How to restore Firefox bookmark with automatic is easy from Bookmark manager that you have known when you want to backup your bookmark and just dig Restore and click the day you want to restore. If you want to use your backup JSON file just click Choose File… 
Next you need to locate your JSON backup file that you have saved previously with manual backup. Next it will provide notification that your backup will replace all your current bookmark and click OK. Maybe you have more than one computer and you want to have same bookmark so you can easily use your backup file to restore in every computer you have. Actually there is another option in Firefox to manage bookmark with import and export html file as backup and restore. Enjoy it!