You can't import a JSON backup in Firefox, but you can restore a JSON backup. Note that restoring a JSON backup replaces all current bookmarks.

You can restore a JSON backup in the Bookmarks Manager (Library).

• Bookmarks -> Show All Bookmarks -> Import & Backup -> Restore

See also:

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

Thanks that worked. I had to do one more thing after that to restore the look I had before: customize the toolbar to show the bookmarks menu.