If you use Gmail you're already familiar with the personal level indicators where a small arrow indicates whether an email was sent to just you, or a group of people. Personal Level Indicators is a small add-on for Thunderbird that adds the same feature to your desktop email client.
The add-on works the exact same way as in Gmail. A single arrow indicates an email was sent to a group. The double arrow indicates it's just for you. It's a handy feature if you're trying to figure out if an email is really just meant for your eyes only if it was sent out to a ton of people. It's also good as a reminder to watch out for the reply all. Personal Level indicators is a free download.
Personal Level Indicators [Mozilla]