User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:93.0) Gecko/20100101 Firefox/93.0

Steps to reproduce:

Make sure you're using the default Dark Theme from Firefox.
Open some tabs.

Actual results:

In the new UI It's not really clear which Tab is the current Tab, specially when you're using the default Dark Theme.
Selected Tab = The tab that the user is working in it.

Expected results:

Having an icon for the current selected tab
or
Making the color of selected tab a bit lighter!
or
Having a big light line bellow the current tab under the Tab's Title