> Firefox vs us on Linux

That's a bit of an unfair comparison. Gtk has lots of nice themes which AFAIK Tk can't use. That dialog doesn't exist on Firefox anymore, but this is what Thunderbird's preferences look like on my machine: https://my.mixtape.moe/fggaex.png

It's true, Tk can look okay on linux, but it will still be out of place when placed side by side with Gtk and Qt apps (Qt can use Gtk themes).