I'm always losing my place when reading long emails, because when I go to scroll down with the up/down cursor keys or page up/down keys, more often than not window focus has switched to one of the list panels without my input and Thunderbird has now switched to a different email or some such, causing me to lose my place in the previous long email that I was trying to read.
I think it'd be really awesome if there was the option to just click a checkbox somewhere and never have Thunderbird ever focus on any other panel than the message view panel, so that this problem never, ever, happens again!