First, a small nitpick - the new Thunderbird icon is a touch smaller than the Firefox icon in the taskbar. It'd be nice if they were the same size for uniformity's sake. I've attached a screenshot illustrating the discrepancy.
Second, and much more pressing, the Get New Messages/Write New Message area is a bit of an eyesore, and clunky to boot. The big blue button doesn't fit any theming... it's just there, sticking out awkwardly. The Get New Messages icon is the reverse... it essentially blends in with the rest of the app's structure. Worse, the functionality to either get all new messages for every email account, or to select which account to get new messages for, is hidden behind a right click. And there's nothing there to suggest those are options at all.
It really feels like there's too much emphasis on "Well, telemetry tells us that people write more new messages than manually checking for messages, so let's make that obvious" and far, far too little on actual aesthetics, flow, balance, etc. Again, the entire area is clunky, and feels like a first pass rather than a finalized design. It's especially jarring that the user cannot customize the button (or, if they can, it's not obvious) to at least make it fit into their system's theming.
In addition to what I wrote above, the upper right side of the app is also not designed very well. I would remove the inexplicable white border from the Quick Filter button, add the lighter gray background to the Message List Display Options button, and move them down a row so they're on the same row as the rest of the normal buttons (but remain shifted right to denote they're different from those buttons).
Similarly, I would move the hamburger menu button down to fill the space vacated by those buttons, and to move it way from the standard window Minimize, Restore/Maximize, and Close buttons.
I've attached a screenshot with some rough indicators of what I'm talking about.