My folder pane on the left of Thunderbird has disappeared, and I'm afraid the recommended solution ("just move your cursor to the far left side of the window until the cursor changes into a double-headed arrow (↔), and then drag the splitter bar to the right") isn't working for me - when I get the double-headed arrow, it just moves the whole window back and forth. (NB. I've tried downloading the latest version of Thunderbird, but it automatically started it up without the folder pane, and I've tried starting in safe mode, with the same result.) Please help! This is a huge problem, leaving me unable to work.