Turns out, the file transfer process was the easiest part. Copied the contents of my home directory to the NAS, thence to the new SSD. Dug down into the .mozilla and .thunderbird directories and copied the .default folders to the NAS. Then copied the .default contents to the .default folders on the new SSD.
That’s it. All my in-the-cloud stuff now available. gMail and Amazon know who I am. Some housekeeping left. Mostly setting up the speed dial on Firefox. Some new installs, like DropBox.
Later today I’ll swap the SSD’s.