Because email standards are if there is only one person you reply. If there are 2 or more you reply all. The default button in Thunderbird is a smart button. It gives you the appropriate reply option. I believe there is a reply all button available in the customize dialog. I am on an Android device so I cannot look right now but you can right click a blank space on the toolbar and select Customize. If the button you want is there drag it to the toolbar.

Other email clients I have used allow Reply all also in this case. So these clients look more at what is practical than what the standard is. It isn't customizable, at least not in the swedish translation. And I guess the translation shouldn't affect what possibilities there are.

Regards, Bengt

I cannot imagine the need to ever reply to myself. If i do want to get a copy i use CC or BCC.

There is an option to always CC or BCC an address in Account Settings but that is always not only for the ones you select

If the other clients you refer to are Microsoft, they make up their own standards.