How do I change profiles and login to my boss' Thunderbird email to help him manage his inbox and settings?

you might just add his account to your existing profile so you have two accounts available in the list of accounts on the left. Or you might want to use a separate profile. What you should not do is share a profile as Thunderbird does not cope well with the same profile being accessed by two different users at the same time. See https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles