There is a header bat at top of message list pane -- such as "subject, from, date, etc...' - To the right is an icon that, if you click it, you can tick which columns to display.

I found that icon, but message preview isn't on there. I couldn't find any way to move message preview from the bottom to the side.

That wasn't your question. Anyway, click view>layout and tick vertical view.

I hope I'm framing this question accurately. The text and photographs in the emails are too small for me to read when I open them. How do I increase the text size and photo size within an email that I received? I already increased the size of text in the pane that shows all the email, but when I open an individual email, I can't read it.

here is link to a post of this:

  https://support.mozilla.org/en-US/questions/1437527    Please follow in detail and it will permanently fix your email font size. It's several steps, but has always worked.