How do I find the correct password to sign in to my Thunderbird email?

It's ambiguous what you're actually looking for. Thunderbird, the email program, doesn't require a password to "sign in". Its only feature that requires a password is Saved Passwords. In other words, if you have that feature enabled, you have to supply a password in order to view the saved passwords. You may be asked for that password when starting Thunderbird, but the password is not required in order use Thunderbird. Or are you asking for the password for your email service (Gmail, Yahoo, AOL, Outlook, etc?) You might be able to see the password in Saved Passwords (look at <Settings | Privacy & Security | Passwords | Saved Passwords>. But if this is an IMAP account, you might not see the actual password, but instead a hashed (encoded) token, which is not the actual password. In that case, you may have to reset your password with the mail provider in order to send and receive.