In order to recover your Firefox 64-bit passwords, you will need to provide a custom Firefox 32-bit Installation Path where it contains the main decryption modules.
This method even works as long as you provide the location to nss3.dll from your older portable version of Firefox 32-bit even without installing it.
Just make sure to check "Use the following Firefox Installation Path" and locate the required decryption modules from the Firefox 32-bit folder.
The image below shows, as an example how doing that.