I tend to really harp on send-in-background because I view it as a core feature of an MUA, but as far as I know it's still hidden behind an about:config flag in Thunderbird because it has an excessive number of known bugs, and it's been this way for many years. Another ongoing pain with Thunderbird is the inability to switch between HTML and plaintext when composing a message. Outlook lets you do this, in Thunderbird you have to copy the message, discard it, start a new one in the right mode and paste.

On the other hand, yes, Outlook Web Access is hilariously bad. It's hard to understand how Microsoft flubbed it so bad considering that their consumer outlook.com has a radically better webmail. Different teams, obviously, but you'd think they would have shared notes.

All in all, it feels a lot to me like Microsoft had a really good MUA in 2003 and hasn't done much with it since, while Mozilla had a not-quite-done-yet MUA in 2003 and hasn't done much with it since. Both feel bolted together out of spare parts but in a way that's subjectively a bit different.