Your professional input may be desired. Though I did not contribute to Thunderbird either (, yet?). Thing is, your critique may result in guidance to make part of these changes according to your skillset! But I am just a hopeless dreamer in the good of many things.
This isn't complex stuff either. I'm not a designer, I don't even do frontend/UI work, I'm basically just a backend engineer. If I'm spotting this stuff on first glance at the main marketing screenshot, which should be the best possible case, then this stuff must be glaringly obvious and pervasive.
I only see one ... icon in the entire screenshot, nothing is fuzzy, and all text looks aligned. The only weirdness is a misalignment between the sidebar header and the main window header, but those don't need to be aligned anyway.