Why are you trying to invoke a debugging session as part of a migration? -allow-downgrade should be sufficient, as you do not get downgrade errors until you have already set the profile to use to an old one. Success of failure will be immediately apparent without a JSconsole or a Debugger My personal choice ATM is to just copy and paste the entire content of the old profile folder and sub folders (select all in the files and folders in file manager when the relevant profile folder is open) into the new one Thunderbird created, commonly with release in the name. It appears to cause the least issues as long as you don't past into the wrong folder. I have seen a number where the profile folder is pasted into a sub folder of the new one for instance. Note the connection refused may well be in response to your anti virus blocking the localhost connection. Some of them are very picky about who and what can use the network adapter loopback address.