I don't particularly hate it (I'm used to it after years and years and the changes have been very progressive, probably) except for the change in the "Get Mail" button. It used to be a big clear button with text, and now it's just a small arrow.

  Old: https://assets-prod.sumo.prod.webservices.mozgcp.net/media/uploads/gallery/images/2021-10-11-13-20-13-5076f8.png
  New (upper left corner) : https://149677182.v2.pressablecdn.com/wp-content/uploads/2022/06/thundebrird-planned-redesign.jpg