Thunderbird Portable User Password Failure

Hi,

I am an amateur user of Fastmail (pc web version ) and Thunderbird Portable (on memory stick). I use the latter to export e-mail from two fastmail accounts using 'ImportExportTools NG'.

I am unable to log in on Thunderbird Portable for one user (lost or forgotten password). I have changed the password for this user in Fastmail ('Users/Aliases', 'Edit', 'Change Password'). However, this new password fails when used in Thunderbird Portable.

Could someone please explain how I can reset or recover the Fastmail user's password so it can be used to access the same user's account in Thunderbird Portable?

Thanks for your help.