I'm attaching a screenshot I took of an email.  I have obscured email addresses and put a small grey line at the bottom of the header, so you will be able to see how much of the screen is taken by a header that won't roll away.

I don't know whether if's Thunderbird itself, or my theme, but the actual incoming email has a hanging header that takes up too much of my screen so that I can only see a couple of lines of text, while keeping the header stationary. I have aging vision and set my text to a large size - so it really impacts my ability to read the message fluently. I have neither dementia or Alzheimer's, so I have no need to have the header constantly in my face while obscuring all but two lines of the message at a time. I spent some time looking at themes and found that very few of them show a sample message screen. And the ones that do, do not indicate whether the header is stationary (rolls up as the text gets scrolled in) or fixed (a hanging header). I'm attaching a screenshot I took of an email. I have obscured email addresses and put a small grey line at the bottom of the header, so you will be able to see how much of the screen is taken by a header that won't roll away.