When you reset your password, any data on Firefox Sync gets erased. This is done because your password is used to create the key that encrypts your data. Once the password is reset, that data can no longer be decrypted and becomes useless.

Unfortunately, if you didn't make local backup of your Firefox data before hand, you are out of luck. Sorry.

Many users have this type of issue. Firefox Sync is not a data backup service and should not be used as one. It doesn't always keep a permanent or complete version of your data, since it's only intended for syncing settings and data between different devices. A local backup of your Firefox profile folder should be made before doing anything to your computer, to ensure that you can completely recover your data.

Thank you. Before I sent my computer in for repair, I asked my computer tech support how to back up my favorites and he told me that syncing would make all my settings and tabs available after the repair.

In some cases it does, but it's not always consistence since the system wasn't designed for data backup. Resetting the password on your Firefox Account also stopped any chance of maybe being able to recovery any Firefox Sync data.

You may want to inform whoever you got technical support from, just to make sure that they don't offer similar advice to future customers.