Please note the following will only work if Thunderbird is running.

First create the email which you want to auto send.

• Click on Write
• Suggest you use the new email address in the From.
• In Subject say something like 'Notice of new email address'.
• compose an email which says that the email address has now changed and state the new email address.
• Then save that email as a Template email in the Templates folder.

Now create a filter:

• Tools > Message Filters
• Click on 'New'
• Give filter a suitable name like 'New Email Address'
• Select 'Getting new mail' and 'Filter after Junk Classification' ( as you do not want it sent to junk mail)

Note the Message Filters get applied to all new incoming mail which arrives in the 'Inbox'.

If you want this sent to every email choose 'Match all messages' then you do not need to set any conditions.

If you want it sent if any of the condition is true then select 'Match any of the following' This is used where you may set more than one condition, but if either is ok then send.

If you want it sent if only all conditions are true select 'Match all of the following' This is used if only one condition is created OR you create more than one condition, but incoming email must meet all conditions.

If you want it only sent to people in your address books set condition:

• 'From' and 'Is in my address book' and select name of address book
• If there are more address books which you need to include then click on the small + to add another line create same as before but selects a different address book.

Perform these actions:

• 'Reply with template' and choose the template email which has the Subject 'Notice of new email address'

• Click on 'OK'

this creates the filter.

• Message Filters get applied in the order listed, so recommend you put the new filter called 'New Email Address' at the top of the list.
• Select the checkbox to enable the filter.

Finally, I suggest you enable the filter log so you have a full record of which emails were sent the reply with template email.

• Click on 'Filter Log' button
• Select the checkbox 'Enable the Filter Log'
• Click on 'Close'

From now onwards any email arriving in the 'Inbox' will be checked against the 'Message Filter' and the reply template email will get applied.

Info on creating Message Filters: https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters

You should be able to set up a 'vacation response' by accessing the account in webmail, so that notifications are sent independent of TB.

https://support.mozilla.org/en-US/kb/vacation-response