Message filters allow you to set up Thunderbird to organize your
messages automatically. Each account has its own set of filters. Filters
can move messages to folders, delete them, forward them to other email
addresses, and more.
When you receive new mail - This is when Getting New Mail is
selected (the default). Filters are applied to new mail in the inbox.
This can occur either before Thunderbird does its assessment of the mail
as junk, or after. If you have difficulty with filtering the message
body, select after classification.
Automatically applied filters are applied only to the inbox, with
the exception of Archive and after send filters, which apply only to the
mails involved in the action to send or archive. Other folder types are
Thunderbird can save passwords for each account, so you don't have
to re-enter them when you check for new messages. You can view them and
remove the ones you don't use anymore.
Do you want to prevent others from seeing your
messages? If you are sharing a computer, the most secure way is
to create a separate OS user account on the operating system for each
person, and make sure your OS account requires a password. Thunderbird
stores accounts, messages and password information separately for each
operating system account.