Your request is not clear. My interpretation is that you are asking to have the sender and recipient the same. More information may help.

Not sure how I could make it more clear. My unified inbox has 4 different inboxes from different email addresses merged into one. When I reply to an email, the reply must be sent from one of those 4 different addresses, but for some reason, thunderbird does not always use the address that the received email was sent to as the default FROM address for the reply like most other email clients do. When you have a linked inbox in outlook or apple mail, this behavior is the default - those clients always use the TO address of the email you received as the FROM address of the email you are replying with. The TO and FROM addresses are never the same on any one email - not sure how you came to that conclusion from what I wrote.

Okay, now I understand. Yes, if you have four unique accounts, and the unified view makes them all appear virtually as one, when you reply, the FROM address should be the one that received the message. Are these each individual accounts, are are they aliases?

I think I may have found the solution (none of my accounts are aliases, they are all unique addresses). For each account, go under account settings and click the header with the address for the account which contains the account settings menu for that account. Then copy and paste the address of that account into the box beside "Reply from this identity when delivery headers match:" and check the box to the left. Doing this for all the accounts and closing and re opening thunderbird seems to have established the reply behavior I'm looking for.

I'm pleased that worked. I'm puzzled, because I do not have that setting, yet my accounts are working as desired. Thanks for sharing.