I've played about with this setting and it looks to me that if "Set as Default" is greyed out it means that Thunderbird already thinks this is the default account.

But what I have also found is that if you have the cursor on the Inbox of one of the other accounts and hit "Write" then it ignores the default you have set and uses the account that you have open.

So I guess that what you are looking for is an option somewhere else that tells "Write" to always use your default rather than the account you happen to have open.

I'll let you know if I find one!

This seems to be the expected behaviour - the help page ([https://support.mozilla.org/en-US/kb/sending-and-receiving-messages-thund...]) just says the following:

If an email (or newsgroup) account is highlighted in the All Folders panel, the message's From address will default to the selected account. If no account is specified, the From address will default to the first account configured in the Account Settings. Click the From drop-down list to select a different account.

And I cannot see an option anywhere that will change this behaviour.

The default applies if your focus is not on any specific account - for example in your local folders - or if are sending a mail from outside Thunderbird - for example from your browser.

I don't want to sound like your manager, but it looks to me that the only solution is to avoid looking at your personal emails while you are working :-)

There is an add-on that helps choose the sending account, no matter which folder is selected:

https://addons.thunderbird.net/en-us/thunderbird/addon/identity-chooser/