Incoming email replies

When I receive an email and send a reply, if another reply is sent to me, it no longer shows as a separate incoming email, but gets placed underneath the original reply with a little arrow next to it. How can I set Thunderbird up to simply show all incoming emails as new arrivals. Under the current setup, it is sometimes several days before I notice that little arrow. If the original email was given the darkened text as new emails get, I would see it. As it is, some folks have sent me new messages because I did not respond to their reply. It's a little embarrassing. I would simply like all incoming email to be shown as individual incoming emails so that I can easily see them. That's how things used to work before the last big update. Thanks for your help, Slider