When I click a folder, the contents that are in this folder does not show up. It only shows a white background. Some times, when I click a folder, it doesn't move at all. Every time this happens, I have to close Thunderbird and restart again. I've tried re-starting my computer and re-installing Thunderbird so many times, but doesn't work! How do I fix this problem? It's so annoying that I have to close and open Thunderbird again whenever I want to move to a different folder...