BACKUP

• Click the menu button to open the menu panel. Click Bookmarks and then click the Manage bookmarks bar at the bottom.
• In the Library window, click the Import and Backup button and then select Backup….
  

• In the Bookmarks backup filename window that opens, choose a location to save the file, which is named bookmarks-"date".json by default. The desktop is usually a good spot, but any place that is easy to remember will work.
• Save the bookmarks json file. The Bookmarks backup filename window will close and then you can close the Library window.

RESTORE

• Click the menu button to open the menu panel. Click Bookmarks and then click the Manage bookmarks bar at the bottom.
• In the Library window, click the Import and Backup button and then select Restore.
  

• Browse to the location, where you saved the bookmarks (JSON) backup file to, then click Open

• A message should pop-up "This will replace all of your current bookmarks with the backup. Are you sure?" Since you likely have not saved any new bookmarks yet, click OK.
  • Note: If you have existing bookmarks on a computer, and you do not want to overwrite them (only add to them), use the Import/Export Bookmarks from HTML… (.html rather than the .json) backup file method. (see links at bottom of page)

• The Bookmarks should now be restored, and available in the browser.

Also see:

Export Firefox bookmarks to an HTML file to back up or transfer bookmarks
https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

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