won't scroll with left mouse button

With the latest Firefox update I've found the when the window is maximized and I have the taskbar set to auto-hide, the screen won't scroll when I hold the left mouse button and pull down. It will scroll up, just not down. It works if I don't auto-hide the taskbar, or if I minimize the screen. Also the mouse wheel scrolls fine. The problem is, when trying to copy a lot of material, I start at the top and hold the left mouse button and try to scroll down to copy all of it, but it won't budge- which it always has until immediately after th latest update. I have a PC with Windows 7.