I wanted to set up an additional email address with my Windows Live Mail account. After setting new address I am not able to get mail from my old address in my Thunderbird email program. It won’t let me sign in. How do I get my old address to work?
Your additional address is tied with your main address, and it's simply an extension. Log in with your main account.
Maybe Microsoft dont let you access 2 account directly from one email client.
How To Set Up Mozilla Thunderbird 3 For Multiple Email Accounts
Importing and exporting your mail
Windows 7 Mail to Thunderbird Import
Switching to Thunderbird
You can't sign in with your additional email address in Windows Live Mail account. Use the old one.