Often I will click "Reply" to someone, it will open a "Compose" window, I will be tying my reply, and the compose window will, without any warning, close - such that all that is displayed is the original message from them. I can find the original message I was composing only by going to the Taskbar, hovering over the Thunderbird icon, and re-opening the "Compose " window. It's the same effect as if I were to minimize the Compose Window - but Thunderbird just randomly does it (while I'm trying my response to the original email). This happens often, and it's VERY annoying.