It turns out that THERE IS ALREADY AN ADD-ON THAT DOES THIS, Show InOut. That add-on is far more functional than this one, so I'm going to stop maintaining it and defer to Show InOut.
If you specifically like the behavior of From/To Column with regards to putting a "to" prefix in front of the recipient for outgoing messages, you can achieve this same functionality in Show InOut by editing these preferences (not available through the preferences GUI, but available through the advanced configuration editor):
- Set "extensions.showInOut.PrefixType" to "text".
- Set "extensions.showInOut.InTextPrefix" and/or "extensions.showInOut.OutTextPrefix" to the prefix you want for inbound and/or outbound messages.