well now, it is hard to answer your question because really I have no idea what you are seeing. Perhaps you might like to post a screen shot of what it actually looks like and what is wrong with it.

I really hate the new metro theme the UI got. Is it possible to get the old UI back?

if you are talking about the square corners on the tabs, not i is not possible to get it back.

So you might want to look at some themes on the add-ons site that might suit you, but I can not see any that use rounded corners that have been updated for use with Thunderbird 60.

know any good open source mail clients that run on windows?

That is what happens with the new versions of Mozilla products. It is the modern design I don't like it either The only way to get it back is to downgrade Not recommended my Mozilla And me

Based on what you said about how you really don't like the blocky design, you can go with IncrediMail, or the integrated Windows Live Mail. You still should stay with Thunder bird because it is the #1 reccomended one by me and dozens of other websites on the internet.

As for rounded tabs, this userChrome.css file works in TB 60:

http://forums.mozillazine.org/viewtopic.php?p=14789900#p14789900