As taken from the Mozilla support site.
Grab the automatic backup of the bookmarks from your old user profile.
C:\Users\USERNAME\AppData\Roaming\Mozilla\Firefox\Profiles\RANDOMCHARACTERS.default\bookmarkbackups\bookmarks-DATE.json
Restoring from backups
Caution: Restoring bookmarks from a backup will overwrite your current set of bookmarks with the ones in the backup file.
Click the Bookmarks button Bookmarks button win 2 on the right side of the navigation toolbar and select Show All Bookmarks to open the Library window.
In the Library window, click the Import and Backup button and then select Restore.
Select the backup from which you want to restore:
The dated entries are automatic backups.
Choose File… lets you restore from a manual backup (see above).
After choosing a backup, your bookmarks from that file will be restored. Close the Library window.
via Restore bookmarks from backup or move them to another computer | Firefox Help.