I switched back to Thunderbird sometime in late 2017, in part because I was working from abroad with spotty Internet at the time. Since coming home, it's stuck. The experience is so much better than waiting for a Gmail tab to "boot up".

I still nip into Gmail quite regularly, especially when I'm already in the browser or a tab is already open, but if I'm sitting at the desktop and need to compose an e-mail, at some point the default returned to alt-tabbing to Thunderbird.

No waiting around, no messing, blank e-mail window up on the screen as fast as I can hit CTRL+N.

Back when everyone switched to Gmail circa 2005ish, hitting CTRL+N quite probably meant a firestorm of random IO hitting a magnetic disk, on a machine with barely any RAM for cache. The equation has long since changed, and your reason to use Gmail might now be your reason to use a desktop client. Highly recommend giving it a shot

One Thunderbird extension I miss is FireTray that let me minimize it to Tray. Number of new emails would show on the icon, so you never missed an email without it also getting in the way of my other work while alt-tabbing.

Birdtray extension (https://github.com/gyunaev/birdtray) does that work quite well, I'm so happy with it. It also lets you keep Thunderbird as a background process.