It’s not hard to figure out, the UI/UX is just kinda ass. It’s got that open source made for developers by developers feel and not made for actual users feel.
The biggest thing I liked about the windows 10 mail client was how well it scaled to different sized displays, and all on the fly. Thunderbird I either make it look good for my 4k monitor, but unusable on my laptop if it’s a small window, or I make it look good on the laptop and it’s horrendous use of space. Without going into crazy themes the thunderbird client looks straight outa 2002 no matter what font sizes and layouts you tweak.