This began a bit ago (few months) and I just didn't have time to deal with it. One day when I tried to drag an image from Windows File Explorer I couldn't get the Thunderbird windows to show after hovering over the Thunderbird tile on the task bar (neither the main program nor the new message would display) and instead it asked if I wanted to pin the image to Thunderbird. When this used to work when I would hover over Thunderbird on the task bar with the image for a second or 2 I would get the mini preview windows. One with the main program and the other would show the new message when I would then hover over the new message it would open the window to full size and I could drop the image into the message.