This has been a frustration for years—I can find posts back to 2012—but it's never been addressed. I like using the Preview Pane so that I can quickly glance at an email to see what it's about, and whether I need to respond to it right away. But I don't consider the Preview Pane to be an official "read", and I don't want previewed emails to be marked as read—not after 10 seconds, not ever. Conversely, when I open an email in its own tab/window, I DO consider that an official "read", and I DO want it marked as read—immediately. But this has never been possible in Thunderbird, and the essential problem is this: the Preview Pane and an opened email are treated as the same action. There is no way to distinguish between them.