https://support.mozilla.org/en-US/questions/776547 https://support.mozilla.org/en-US/questions/1099128 I've tried the suggestion from above links, but none solve my problem. My Windows7 Taskbar appearance "Lock the taskbar" is checked. Other maximized window's program is not hiding the taskbar, normal. The way Firefox hides my taskbar is unpredictable : A. open Firefox in maximized window, the taskbar is not seen B. open Firefox in maximized window, the taskbar is seen.... but when I want to click the taskbar by moving the mouse pointer to the taskbar, as soon as the pointer arrive at the taskbar (not clicked yet), the taskbar disappear. C. open Firefox in maximized window, the taskbar is seen.... and I can click the taskbar (the empty one). But as soon as I click the Firefox page, the taskbar disappear. The problem is since yesterday. Yesterday I thought "it will be back to normal after I shutdown the computer today and turn it on tomorrow" :). Any kind of help would be greatly appreciated. Thank in advanced.