Sync isn't a backup it does just what it says sync data if you sync new data old data is wiped out. And if you did no Firefox backups there nothing any user on here can help. Reset wipes clean everything.

But I still have my old Windows 10 hard drives. So I was wondering if anyone knew what folder to look under on my old hard drives that might have Firefox bookmarks? That was my question. I'm aware the new hard drive has been wiped. But I still have 2 old hard drives that might have my bookmarks on them. Because I didn't wipe or reset or sync those drives.

Brian, Firefox maintains backups of bookmarks in a folder named "bookmarkbackups" within a profile folder. So you could try searching the old drives for folders with this name.

The backups will have names of the form "bookmarks-*.jsonlz4". So alternatively, you could try searching for filenames matching this pattern.

If you find any such backups, then you can import them via the "Import and Backup" menu in the bookmarks library. See the "Restoring from backups" section here: https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them (But note the part "Caution: Restoring bookmarks from a backup will overwrite your current set of bookmarks with the ones in the backup file".)