use a specific email profile as default

I suppose you mean email account. Profile in the context of Thunderbird means something entirely different. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data

Please make sure to use the proper terminology.

Default account in Thunderbird basically means it's the one on the top of the folder pane. When composing a new message or responding to an existing one Thunderbird uses the From: email address of the account currently selected.

If this doesn't fit you can change the From: email to be used from the drop-down in the From: field.

Christ1,

Thanks for the fast response. And yes, I will now be as 100% accurate as I can be as we are back in the world of software development and we must be 100% specific.

first, Yes, I mean account, so we are set there.

second, when you say "Default account in Thunderbird basically means it's the one on the top of the folder pane." let's not quibble about terminology here when you say "folder" because as you can see, this is not about folders, as in the account settings section of Tools--->Account Settings, I have specified the account to be used in green, and it is in the 1st position HOWEVER when I respond or create a new message this is NOT the default account that is used in the message and it should be

third when you say "Thunderbird uses the From: email address of the account currently selected." ---> currently "selected" WHERE ? In the accounts section above? There is even a way to select which account you want to use in the "Account Actions" button below all the accounts and I have, properly, selected the 1st account in the list ( in attached image ) that is in greed and set it as the default account and STILL it will not use that when composing or responding to a message.

What else can be done? This can't be normal operating functionality can it ?

If you receive a message addressed to your green account, and you reply to it, Thunderbird's default behaviour would be to reply using an identity associated with the green account. If you have any folder in the green account open and selected for viewing, then any new message you write will use an identity associated with the green account.

Replace "green" by "red" and the same thing happens; a reply to a message addressed to your red account should use an identity associated with the red account. A new message will use an identity associated with the red account.

If you're getting messages in the green account and want to reply from the red account, you should look at add-ons designed to help with automatic or prompted selection of the correct account. Add-ons I have used are Correct Identity, Folder Accounts and Identity Chooser.

The default account only comes in to play when you create a new message and there is nothing else to determine which identity would be appropriate to use. Examples are that you are currently browsing the Local Folders account (which doesn't have an identity), or you launched Thunderbird from an external "send to…" or mailto: link.

Ahhhhhhhhhh Zenos, and THERE it is. When a program says "Set As Default" you would think that means, THE default for mail activities NOT the default based on what FOLDER you are in. I have no idea why the developers at Thunderbird/Modzilla think that "default" does not, in fact mean, THE one, THE default, that is used, but there is always something to learn in life.

In this case, at least to me, it is now clear, DEFAULT is not default because it seems that whatever FOLDER you are in, THAT overrides the default ( in green ) that is set. This is clearly demonstrated by this example. IF i am in the folder of the RED account, then if I decide to make a "new message or reply to a message there, THIS ( the red account details ) are the account details ( reply to address, reply address ) that is used for said reply. IF i am in the green account, then the same applies ( respectively ) but ONLY if I am in that ( green ) folder.

While I can understand why developers would set things up as they have, you ALSO might want to use the DEFAULT set in the account settings for any messages, IF, like me, you have ALL your messages going to ONE folder ( so that there is no switching ) around and I never have to touch the mouse while I'm flying through emails. I do this , so I can essentially just look at everything that is coming in and out of my life ( ya, poor work/life balance there, I know ) but at least I can see everything in one shot and move quickly.

Might there be some special setting that can be forced so that whenever a reply or email is created, it automatically pulls the details from the DEFAULT account set in the Tools ---> Account Settings section. For me, work things ( for whatever reason ) sometimes come through my personal account and I'd like to have them be responded to by my work email address & settings.

Thanks in advance

True. It is the default when there is nothing else to determine which email account (or address, i.e. alias, etc). This is exactly what happens (uses default) when you have your FOLDERS in the FOLDERS PANE set to Unified Folders. You can see/set that option if you have the View, Layout, Folder Pane checked. Then when you are viewing All your Inboxes in the Unified Folder View, in the Folder Pane, and you click on New Message (or use Menu, Message, New Message...Your New Message will be from your Default Email Account that you have chosen.

Zenos said

The default account only comes in to play when you create a new message and there is nothing else to determine which identity would be appropriate to use. Examples are that you are currently browsing the Local Folders account (which doesn't have an identity), or you launched Thunderbird from an external "send to…" or mailto: link.

I listed for you the add-ons I use to manage the business of choosing which identity to use.

Funnily enough, at least one of these add-ons claims that Thunderbird's default behaviour, when replying, is to pick out the address to which the incoming message was sent. I wonder why this isn't working for you.

As lindalou has commented, use of Unified Folders removes the "selected account" notion, so Thunderbird ought to be replying to the address the message was sent to. But it appears under these circumstances it reverts to the default account.

However the very existence of the many add-ons for managing identities indicates that the automatic thing doesn't work reliably. :-(