Would you have accidentally deleted your key pairs, I am afraid that there is no way to get it back from Thawte. Indeed, for security reasons, they only allow keys to be downloaded once.
You can however use your backup file from Firefox to re-import them into the Keychain.
Would you no longer have this file, you may have to revoke your existing certificates and get new ones. Just keep in mind that this may prevent you from opening any encrypted mails you may have in your mailboxes.
Let me know if this helps!