I don't know for sure what you see as the problem, so I will, instead, explain how it works for me. - I set an alias email id - I send email to a person, using the alias - the person's response comes into my primary email account to which the alias is a managed identity. - when I click 'reply', the alias is shown as the sender, not my primary email address.

That is, when a message arrives into an account, Thunderbird remembers that the message was sent to an alias and uses that by default on responses to the other party.

I've no idea what you mean by "alias." So I'll explain further... I have 3 email accounts set up in TB. Under the first, let's call it first@aaa.net, I have several identities set up which include several gmail account addresses, and first@aaa.net is clearly marked as the default. The other two accounts have no additional identities and neither is marked as the default.

TB never chooses any of the email account addresses or any address of my identities, used by the sending author. It ALWAYS forces me via a pop up box to select an address to use in my responses. Actually, it does show in the top line, From:, the identity to which the original email was addressed, however, I must nonetheless select ("Please select an identity to proceed:") the address I wish to use before I can even start composing my response. This annoying failure began maybe a few months ago; it ticks me off and wastes my time. If it would simply always use my default account/email address, first@aaa.net, I could survive. Then I would only occasionally have to select which address to use in my response. But to be forced to ALWAYS select a responding address wastes my time and I find extremely irritating.

So, you're saying that when a person writes to you at first@aaa.net and you click reply, that you must first specify from which address you're replying from? Please clarify that, as Thunderbird (for me) has never worked that way. Maybe I'm missing something here. As an aside, the word 'identities' is used with Thunderbird to identify additional email addresses that are affiliated with a primary address. These are also called aliases.

Correct: when a person writes to me at first@aaa.net and I click reply, I must first specify in a drop down box from which address I am replying. This happens no matter which of my email addresses receives emails including my other two account email addresses, second@aaa,net, third@aaa.net, and all my identities@gmail.com. The email address used by the sender does appear at the top next to "From:" however, that drop down box titled with "Please select an identity to proceed:" forcing me to select the address I want to use to respond always appears before I can do anything more.

Does that dialog look like this, as Thunderbird does not have an identity chooser of it's own. addons.thunderbird.net/user-media/previews/thumbs/207/207092.png?modified=1691278809

I am guessing you have installed the identity chooser addon https://addons.thunderbird.net/en-US/thunderbird/addon/identity-chooser/?src=ss and are here complaining about it's dialog asking you to choose,just disable it in the addon manager.

As I don't use any identity related addons I am just trying to figure out how you get a dialog of that kind and assume you have that one. But it could be one of the others. https://addons.thunderbird.net/en-US/thunderbird/search/?q=identity&appver=115&platform=

"Does that dialog look like this, as Thunderbird does not have an identity chooser of it's own. addons.thunderbird.net/user-media/previews/thumbs/207/207092.png?modified=1691278809 YES!

The identity chooser turned out to be the ugly culprit. I don't recall adding it on, but with your kind guidance, I deactivated it and the annoyance went away.

Thank you.