Thunderbird only has passwords and user names for the email accounts you have given it. so if you have forgotten a password, you can look up what Thunderbird has stored, but to change it you need to go back the the supplier of the email account. There are no reset or reminder emails for Thunderbird at all. Most of those making posts here like yours are requesting their passwords for this site be reset.

Thank you for your response. I tried the sign on I have always used and they were not working. Then all of a sudden Mozilla let me in without the sign on or password. WHY????

It has nothing to do with 'Mozilla' letting you in.

It is not possible to logon to any server without a username and password. If you are not entering password then Thunderbird is using the one you told it to remember when setting up the account. Thunderbird will use those stored passwords when it needs to access the relevant server to download/send emails.

It is the server which may choose to not recognise eg: password, but there could be many reasons for this.

If you ever update the password in the webmail account, then you would need to update the stored password. The server may have been temporarilly busy or other issue. Sometimes you need to logon to webmail in order to get the server accepting everything and sometimes it is a setting that needs to be adjusted in the webmail account settings. Sometimes, your Firewall will block access not allowing Thunderbird to access the internet. This can occur after an update and the Anti-Virus product has not recognised the new updated Thunderbird as the same as the old one.