NO NO NO. Thunderbird is an email client, it should look and work like an email client. Yes the functionality could be improved (as mentioned below search has some issues) but this isn't a UI issue.

Why programs insist on an UI update every so often - well commerical programs to convince you to buy a new license.

Tool makers don't go, every few years "hey we have this spanner, it looks like a spanner, works well as a spanner, lets redesign it so it no longer looks like a spanner, doesn't work as good as the old spanner but maybe we can convince people to buy it again"