This is the only real downside of Thunderbird as far as I’m concerned, other things like aesthetics just being matters of taste where you can’t please everyone. It’s become an annoying downside for me lately because one of my email addresses now only supports the web client and Exchange; no more IMAPS. I really cannot find another email client on Linux that works as well as Thunderbird though, at least not one that also supports Exchange, so Thunderbird it is. The new UI in OP looks great too.