If you change your sync key, all your data will be erased from the sync server.

Yeah, that screws me. How can I find out what my sync key is? What's the point of having a user name and password if it isn't going to let you access your own data? Is there anyway to find out from Mozilla what my sync key is?

The sync key is used to encrypt and decrypt the data going to and coming from the sync server. The sync key is stored on every device that has been connected to that sync account. It isn't stored anywhere else, especially on the Mozilla sync server - no one else has access to your data in an unencrypted format when only you have the "key".

Wasn't your old hard drive returned to you after installation of the new drive? Pop it into a USB enclosure and see if you can recover your Firefox Profile data.
https://support.mozilla.com/en-US/kb/Recovering+important+data+from+an+old+profile
You need the key3.db & signons.sqlite files, specifically, to retrieve your sync key.